Grzimek's Animal Life Encyclopedia.— Second Edition, 17 Volumes, series editor: Michael Hutchins, [Volume 1. Lower Metazoans and Lesser Deuterosomes / Neil Schlager, Editor — Volume 2. Protostomes / Neil Schlager, Editor — Volume 3. Insects / Neil Schlager, Editor — Volume 4-5. Fishes I-II; Fishes I: II: / Neil Schlager, Editor — Volume 6. Amphibians / Neil Schlager, Editor — Volume 7. Reptiles / Neil Schlager, Editor— Volume 8-11. —Birds I-IV; Birds I: II: III: IV: /Donna Olendorf, Editor— Volume 12-16. Mammals I-V; Mammals: I: II: III: IV: V:/ Melissa C. McDade, Editor — Volume 17. Cumulative Index / Melissa C. McDade, Editor] Sponsored by American Zoo and Aquarium Association (Gale Group) This title is also available as an e-book. For a rather lush scientific survey of the better known animal species, no other reference collects such a sheer wealth of systematic data. Besides providing a general view of the set as a whole, we also provide each major animal group with its introductory chapter that reasonably well demonstrates the scope of this reference work. Grzimek's Animal Life Encyclopedia, state of the art taxonomy and naturalism, provides both generic, introductory information as well as species descriptions. The are chapters written well enough for the general scientific reader. No college or university library should be without this encyclopedic survey.
Grzimek's Animal Life Encyclopedia.— Second Edition s an internationally prominent scientific reference compilation, first published in German in the late 1960s, under the editorship of zoologist Bernhard Grzimek (1909-1987). In a cooperative effort between Gale and the American Zoo and Aquarium Association, the series is being completely revised and updated for the first time in over 30 years. Gale is expanding the series from 13 to 17 volumes, commissioning new color images, and updating the information while also making the set easier to use.
General organization by volume is by order and family: The overall structure of this reference work is based on the classification of animals into naturally related groups, a discipline known as taxonomy‑the science through which various organisms are discovered, identified, described, named, classified, and catalogued. Starting with the simplest life forms, the protostomes, in Vol. 1, the series progresses through the more complex animal classes, culminating with the mammals in Vols. 12-‑16. Volume 17 is a stand‑alone cumulative index.
Organization of chapters within each volume reinforces the taxonomic hierarchy.
Opening chapters introduce the class of animal, followed by chapters dedicated
to order and family. Species accounts appear at the end of family chapters. To
help the reader grasp the scientific arrangement, each type of chapter has a
distinctive color and symbol:
Black triangle symbol: Family Chapter (yellow background)
Black circle symbol: Order Chapter (blue background)
White triangle inside black circle symbol: Monotypic Order Chapter (green background)
As chapters narrow in focus, they become more tightly formatted. General chapters have a loose structure, reminiscent of the first edition. While not strictly formatted, order chapters are carefully structured to cover basic information about member families. Monotypic orders, comprised of a single family, utilize family chapter organization. Family chapters are most tightly structured, following a prescribed format of standard rubrics that make information easy to find and understand. Family chapters typically include:
Thumbnail introduction: Common name, Scientific name, Class, Order, Suborder,
Family, Thumbnail description, Size Number of genera, species, Habitat,
Main essay: Evolution and systematics, Physical characteristics, Distribution, Habitat, Behavior, Feeding ecology and diet, Reproductive biology, Conservation status, Significance to humans
Species accounts: Common name, Scientific name, Subfamily, Taxonomy, Other common names, Physical characteristics, Distribution, Habitat, Behavior, Feeding ecology and diet, Reproductive biology, Conservation status, Significance to humans
Resources, Books, Periodicals, Organizations, Other
Editors Introduction to the Second edition: Earth is teeming with life. No
one knows exactly how many distinct organisms inhabit our planet, but more than
5 million different species of animals and plants could exist, ranging from
microscopic algae and bacteria to gigantic elephants, redwood trees and blue
whales. Yet, throughout this wonderful tapestry of living creatures, there runs
a single thread: Deoxyribonucleic acid or DNA. The existence of DNA, an elegant,
twisted organic molecule that is the building block of all life, is perhaps the
best evidence that all living organisms on this planet share a common ancestry.
Our ancient connection to the living world may drive our curiosity, and perhaps
also explain our seemingly insatiable desire for information about animals and
nature. Noted zoologist, E.O. Wilson, recently coined the term "biophilia" to
describe this phenomenon. The term is derived from the Greek bios meaning "life"
and philos meaning "love."
Our own personal awakenings to the natural world are as diverse as humanity
itself. I spent my early childhood in rural
I consider myself very fortunate to have made a living as a professional biologist and conservationist for the past 20 years. I've traveled to over 30 countries and six continents to study and photograph wildlife or to attend related conferences and meetings. Yet, each time I encounter a new and unusual animal or habitat my heart still races with the same excitement of my youth. If this is biophilia, then I certainly possess it, and it is my hope that others will experience it too. I am therefore extremely proud to have served as the series editor for the Gale Group's rewrite of Grzimek's Animal Life Encyclopedia.— Second Edition, one of the best known and widely used reference works on the animal world. Grzimek's is a celebration of animals, a snapshot of our current knowledge of the Earth's incredible range of biological diversity. Although many other animal encyclopedias exist, Grzimek's Animal Life Encyclopedia.— Second Edition remains unparalleled in its size and in the breadth of topics and organisms it covers.
The revision of these volumes could not come at a more opportune time. In fact,
there is a desperate need for a deeper understanding and appreciation of our
natural world. Many species are classified as threatened or endangered, and the
situation is expected to get much worse before it gets better. Species
extinction has always been part of the evolutionary history of life; some
organisms adapt to changing circumstances and some do not. However, the current
rate of species loss is now estimated to be 1,000‑10,000 times the normal
"background" rate of extinction since life began on Earth some 4 billion years
ago. The primary factor responsible for this decline in biological diversity is
the exponential growth of human populations, combined with peoples'
unsustainable appetite for natural resources, such as land, water, minerals,
oil, and timber. The world's human population now exceeds 6 billion, and even
though the average birth rate has begun to decline, most demographers believe
that the global human population will reach 810 billion in the next 50 years.
Much of this projected growth will occur in developing countries in Central and
Finding solutions to conservation challenges will not be easy in today's human‑dominated world. A growing number of people live in urban settings and are becoming increasingly isolated from nature. They "hunt" in super markets and malls, live in apartments and houses, spend their time watching television and searching the World Wide Web. Children and adults must be taught to value biological diversity and the habitats that support it. Education is of prime importance now while we still have time to respond to the impending crisis. There still exist in many parts of the world large numbers of biological "hotspots"‑places that are relatively unaffected by humans and which still contain a rich store of their original animal and plant life. These living repositories, along with selected populations of animals and plants held in professionally managed zoos, aquariums and botanical gardens, could provide the basis for restoring the planet's biological wealth and ecological health. This encyclopedia and the collective knowledge it represents can assist in educating people about animals and their ecological and cultural significance. Perhaps it will also assist others in making deeper connections to nature and spreading biophilia. Information on the conservation status, threats and efforts to preserve various species have been integrated into this revision. We have also included information on the cultural significance of animals, including their roles in art and religion,
It was over 30 years ago that Dr. Bernhard Grzimek, then director of the
Frankfurt Zoo in
Professor Grzimek has recently been criticized by some for his failure to
consider the human element in wildlife conservation. He once wrote: "A national
park must remain a primordial wilderness to be effective. No men, not even
native ones, should live inside its borders." Such ideas, although considered
politically incorrect by many, may in retrospect actually prove to be true.
Human populations throughout
While it is clear that modern conservation must take the needs of both wildlife and people into consideration, what will the quality of human life be if the collective impact of short-term economic decisions is allowed to drive wildlife populations into irreversible extinction? Many rural populations living in areas of high biodiversity are dependent on wild animals as their major source of protein. In addition, wildlife tourism is the primary source of foreign currency in many developing countries and is critical to their financial and social stability. When this source of protein and income is gone, what will become of the local people? The loss of species is not only a conservation disaster; it also has the potential to be a human tragedy of immense proportions. Protected areas, such as national parks, and regulated hunting in areas outside of parks are the only solutions. What critics do not realize is that the fate of wildlife and people in developing countries is closely intertwined. Forests and savannas emptied of wildlife will result in hungry, desperate people, and will, in the longterm lead to extreme poverty and social instability. Dr. Grzimek's early contributions to conservation should be recognized, not only as benefiting wildlife, but as benefiting local people as well.
Dr. Grzimek's hope in publishing his Animal Life Encyclopedia was that it would "...disseminate knowledge of the animals and love for them", so that future generations would "...have an opportunity to live together with the great diversity of these magnificent creatures." As stated above, our goals in producing this updated and revised edition are similar. However, our challenges in producing this encyclopedia were more formidable. The volume of knowledge to be summarized is certainly much greater in the twenty‑first century than it was in the 1970's and 80's. Scientists, both professional and amateur, have learned and published a great deal about the animal kingdom in the past three decades, and our understanding of biological and ecological theory has also progressed. Perhaps our greatest hurdle in producing this revision was to include the new information, while at the same time retaining some of the characteristics that have made Grzimek's Animal Life Encyclopedia.— Second Edition, so popular. We have therefore strived to retain the series' narrative style, while giving the information more organizational structure. Unlike the original Grzimek's, this updated version organizes information under specific topic areas, such as reproduction, behavior, ecology and so forth. In addition, the basic organizational structure is generally consistent from one volume to the next, regardless of the animal groups covered. This should make it easier for users to locate information more quickly and efficiently. Like the original Grzimek's, we have done our best to avoid any overly technical language that would make the work difficult to understand by non‑biologists. When certain technical expressions were necessary, we have included explanations or clarifications.
Considering the vast array of knowledge that such a work represents, it would be impossible for any one zoologist to have completed these volumes. We have therefore sought specialists from various disciplines to write the sections with which they are most familiar. As with the original Grzimek's, we have engaged the best scholars available to serve as topic editors, writers, and consultants. There were some complaints about inaccuracies in the original English version that may have been due to mistakes or misinterpretation during the complicated translation process. However, unlike the original Grzimek's, which was translated from German, this revision has been completely re‑written by English-speaking scientists. This work was truly a cooperative endeavor, and I thank all of those dedicated individuals who have written, edited, consulted, drawn, photographed, or contributed to its production in any way. The names of the topic editors, authors, and illustrators are presented in the list of contributors in each individual volume.
The overall structure of this reference work is based on the classification of animals into naturally related groups, a discipline known as taxonomy or biosystematics. Taxonomy is the science through which various organisms are discovered, identified, described, named, classified and catalogued. It should be noted that in preparing this volume we adopted what might be termed a conservative approach, relying primarily on traditional animal classification schemes. Taxonomy has always been a volatile field, with frequent arguments over the naming of or evolutionary relationships between various organisms. The advent of DNA fingerprinting and other advanced biochemical techniques has revolutionized the field and, not unexpectedly, has produced both advances and confusion. In producing these volumes, we have consulted with specialists to obtain the most up‑to‑date information possible, but knowing that new findings may result in changes at any time. When scientific controversy over the classification of a particular animal or group of animals existed, we did our best to point this out in the text.
Readers should note that it was impossible to include as much detail on some animal groups as was provided on others. For example, the marine and freshwater fish, with vast numbers of orders, families, and species, did not receive as detailed a treatment as did the birds and mammals. Due to practical and financial considerations, the publishers could provide only so much space for each animal group. In such cases, it was impossible to provide more than a broad overview and to feature a few selected examples for the purposes of illustration. To help compensate, we have provided a few key bibliographic references in each section to aid those interested in learning more. This is a common limitation in all reference works, but Grzimek's Encyclopedia of Animal Life is still the most comprehensive work of its kind.
Grzimek's features approximately 3,500 color photos, including approximately 480 in four Birds volumes; 3,500 total color maps, including almost 1,500 in the four Birds volumes; and approximately 5,500 total color illustrations, including 1,385 in four Birds volumes. Each featured species of animal is accompanied by both a distribution map and an illustration.
All maps in Grzimek's were created specifically for the project by XNR Productions. Distribution information was provided by expert contributors and, if necessary, further researched at the University of Michigan Zoological Museum library. Maps are intended to show broad distribution, not definitive ranges, and are color coded to show resident, breeding, and nonbreeding locations (where appropriate).
All the color illustrations in Grzimek's were created specifically for the project by Michigan Science Art. Expert contributors recommended the species to be illustrated and provided feedback to the artists, who supplemented this information with authoritative references and animal skins from University of Michgan Zoological Museum library. In addition to species illustrations, Grzimek's features conceptual drawings that illustrate characteristic traits and behaviors.
About the contributors
The essays were written by expert contributors, including ornithologists, curators, professors, zookeepers, and other reputable professionals. Grzimek's subject advisors reviewed the completed essays to insure that they are appropriate, accurate, and up‑to‑date.
In preparing these volumes, the editors adopted a conservative approach to taxonomy, relying primarily on Peters Checklist (1934‑1986)‑a traditional classification scheme. Taxonomy has always been a volatile field, with frequent arguments over the naming of or evolutionary relationships between various organisms. The advent of DNA fingerprinting and other advanced biochemical techniques has revolutionized the field and, not unexpectedly, has produced both advances and confusion. In producing these volumes, the publisher consulted with noted taxonomist Professor Walter J. Bock as well as other specialists to obtain the most up‑to‑date information possible. When scientific controversy over the classification of a particular animal or group of animals existed, the text makes this clear.
Grzimek's has been designed with ready reference in mind and the editors have standardized information wherever feasible. For Conservation status, Grzimek's follows the IUCN Red List system, developed by its Species Survival Commission. The Red List provides the world's most comprehensive inventory of the global conservation status of plants and animals. Using a set of criteria to evaluate extinction risk, the IUCN recognizes the following categories: Extinct, Extinct in the Wild, Critically Endangered, Endangered, Vulnerable, Conservation Dependent, Near Threatened, Least Concern, and Data Deficient. For a complete explanation of each category, visit the IUCN web page at http://www.iucn.org/themes/ssc/redlists/categor.htm
In addition to IUCN ratings, essays may contain other conservation information, such as a species' inclusion on one of three Convention on International Trade in Endangered Species (CITES) appendices. Adopted in 1975, CITES is a global treaty whose focus is the protection of plant and animal species from unregulated international trade.
Grzimek's provides the following standard information on avian lineage in
Taxonomy rubric of each Species account: [First described as] Muscicapa rufitons
[by] Latham, [in] 1801, [based on a specimen from]
Other common names in English, French, German, and Spanish are given when an accepted common name is available
Appendices and index
For further reading directs readers to additional sources of information about birds. Valuable contact information for Organizations is also included in an appendix. While the encyclopedia minimizes scientific jargon, it also provides a Glossary at the back of the book to define unfamiliar terms. An exhaustive Aves species list records all known species of birds, categorized according to Peters Checklist (1934‑1986). And a full‑color Geologic time scale helps readers understand prehistoric time periods. Additionally, each of the multivolume subsets contain their own full subject index.
Lower Metazoans and Lesser Deuterosomes edited by Neil Schlager (Gale Group)
Excerpt: The Metazoa, today taken as a synonym of Animalia, comprises a large
grouping of organisms that may be characterized as being multicellular and
heterotrophic, i.e., they do not synthesize their own food, but obtain it from
external sources. While there has been some debate in the past, it now seems
overwhelmingly likely that the metazoans are monophyletic, and thus that all
living examples are descended from an animal that lived some time in the
Proterozoic (probably at least 600 million years ago [mya]). Even this statement
is controversial, however. Molecular evidence suggests that metazoans had
emerged up to one billion years ago or more; but the fossil record is most
reasonably read as implying a much later origin, with definitive metazoans not
appearing before 600 mya, and perhaps even later. Unfortunately, most "lower"
metazoans today lack substantial hard parts such as mineralized shells, so their
fossil record is correspondingly very poor. Telling the true time of appearance
of such animals just from the fossil record may therefore well be inaccurate.
Lower Metazoans and Lesser Deuterosomes edited by Neil Schlager (Gale Group)
Excerpt: The Metazoa, today taken as a synonym of Animalia, comprises a large
grouping of organisms that may be characterized as being multicellular and
heterotrophic, i.e., they do not synthesize their own food, but obtain it from
external sources. While there has been some debate in the past, it now seems
overwhelmingly likely that the metazoans are monophyletic, and thus that all
living examples are descended from an animal that lived some time in the
Proterozoic (probably at least 600 million years ago [mya]). Even this statement
is controversial, however. Molecular evidence suggests that metazoans had
emerged up to one billion years ago or more; but the fossil record is most
reasonably read as implying a much later origin, with definitive metazoans not
appearing before 600 mya, and perhaps even later. Unfortunately, most "lower"
metazoans today lack substantial hard parts such as mineralized shells, so their
fossil record is correspondingly very poor. Telling the true time of appearance
of such animals just from the fossil record may therefore well be inaccurate.
Because of their long history and enormous adaptability, animals are organized in a remarkable number of different ways, ranging from simple sponges with only a few cell types through to the vertebrates with their complex nervous and immune systems. Indeed, it is possible to arrange animals in a broad series, from organisms that do not possess true tissues (e.g., sponges), through organisms with tissues but no organs (e.g., cnidarians), into the bilaterally symmetrical animals (the Bilateria) with both. The Bilateria typically also possess a central nervous system and muscles; some of them are segmented; and some possess a body cavity called a coelom.
The evolution of the animals has long been a contentious issue that has generated a huge number of theories. Nevertheless, at the heart of the issue is whether or not the organizational gradient that can be erected tells anything at all about animal evolution, or whether it merely reflects different adaptive needs of each organism. To put it more simply: are all the simple animals more basal than the more complex ones, or is animal evolution less tidy than that? The traditional assumption has been that organization is indeed a reflection of animal relationships and evolution, although the more thoughtful authors have refrained from definitively stating this. In this view, it makes sense to talk about animal evolution being a more or less stately progress from simple to complex; thus, one can label the simple animals, which are at the bottom of the tree, the "lower" metazoans, and the more advanced ones, the "higher" metazoans. To be more precise, animals without coeloms or segments are typically thought of as being "lower." These sorts of organisms show a variety of functional adaptations. Sponges and cnidarians typically have some sort of central fluid-filled cavity, which is critical to many roles including support, nutrition, excretion, and re-production. Small bilaterians, on the other hand, have typically no need of any such system, as they are small enough to allow diffusion directly to and from the body tissues. Although lower metazoans by the definition here lack a true coelom, which can be used as a hydrostatic skeleton, such a tack is re-placed either by the use of a rather solid array of muscles, or by some other type of body cavity, such as the so-called pseudocoelom. It should be stressed that this "lower" terminology is a remnant of certain types of eighteenth century views of the world that are in many ways entirely inappropriate to the modern evolutionary ways of thinking about animals. Furthermore, modern systematic practice forbids the use of taxonomic units that are defined by exclusion—lower metazoans defined by being everything except the higher metazoans, which are usually taken as deuterostomes, arthropods, mollusks, and perhaps annelids, together with their close allies. Nevertheless, the central issue of the relationship of overall form to evolution remains unsolved, and, indeed, has been in hot contention since the last years of the twentieth century.
has become sharp because of the introduction of entirely new sources of data
that have bearing on the problem, i.e., evidence from molecules. Analysis of
the nucleic acids allows a view of animal evolution that is completely, or
largely, independent from that provided by classical morphological studies, and
the results have sometimes been surprising. The sponges, with their relatively
poorly organized morphology, remain basal within the tree, followed by the
cnidarians (jellyfish, corals, and allies) and the ctenophores (the comb
jellies), although the exact relationships between these three is contentious.
All other animals fall into the Bilateria, but the relationships within this
group remain highly debated. On a strictly "progressionist" view, the most basal
bilaterians would be the flatworms, followed by animals that possess a body
cavity that is not fully bounded by mesodermally derived epithelium (i.e., the
coelom), followed by the coelomates themselves. However, this view, prevalent
among scientists in
The advent of molecular systematics has had a dramatic effect on the view of animal relationships, especially within the Bilateria. Two important features stand out. First, the group of lower metazoans referred to collectively as pseudocoelomates, possessing a body cavity that does not fall under the definition of the coelom, is seen to be a highly heteregeneous group. The most surprising aspect of their reassignment has been the proposal that some of them, notably the nematodes and priapulids, are close relatives to the arthropods (forming the Ecdysozoa), displacing the annelids that are traditionally placed in this position. Secondly, at least some of the flatworms have been largely displaced from the base of the tree to form a group loosely related to the annelids and mollusks in a remnant of one of the old branches of the bilaterians, the protostomes. Both these developments lend support to the idea thatthe ancestral bilaterians were rather complex. Nevertheless, at least some of the flatworms, the acoels, have now been rein-stated at the base of the tree. Are their simple features primitive for all of the bilaterians, or have they simply lost their complex features, as have the other flatworms by implication of their higher position? One further line of evidence has come from the shared developmental mechanisms between the bilaterians. Genes that control the layout of the complex morphology, including segments, eyes, and the development of the heart, are all highly conserved between the "higher" metazoans such as the arthropods and chordates. The implication is that these genes are present in the lower bilaterians (and indeed, this has been shown in some cases), and that they may originally have functioned as they do today—implying the very deep origin of the structures these genes now regulate. Such conclusions are controversial, and have by no means been accepted by all, especially morphologists.
metazoans comprise animals that are thought to lie relatively close to the
evolutionary root of animals as a whole. While they all have their own
specializations, it is possible, and to be hoped for, that they retain some
features that were characteristic of the very earliest stages of metazoan and
bilaterian evolution. Although it is widely accepted that the sponges are basal
within the animals, broadly followed by the ctenophores and cnidarians, the
relationships of the bilaterians remain highly controversial. Molecular
systematics places the bilaterians into three groups: the Deuterostomia
(including, among others, echinoderms and chordates); the Ecdysozoa
(principally arthropods and nematodes), and the Trochozoa (annelids, mollusks,
most flatworms, and several other small groups, including the brachiopods). At
least one group of the flatworms may still be basal within the bilaterians, but
the implications this position may have for the evolution of the lower
metazoans are presently unclear. The fundamental questions about bilaterian
evolution, including the origin of the coelom and segmentation, remain
unanswered, and, indeed, as vigorously contested as ever.
The term Protostomia (from the Greek "proto," meaning first, and "stoma," meaning mouth) was coined by the biologist Karl Grobben in 1908. It distinguishes a group of invertebrate animals based upon the fate of the blastopore (the first opening of the early digestive tract) during embryonic development. Animals in which the blastopore becomes the mouth are called protostomes; those in which the mouth develops after the anus are called deuterostomes (from the Greek "deutero," meaning second, and "stoma," meaning mouth).
Protostomia and Deuterostomia are considered superphyletic taxa, each containing a variety of animal phyla. Traditionally, the protostomes include the Annelida, Arthropoda, and Mollusca, and the deuterostomes comprise the Echinodermata and Chordata. Grobben was not the first biologist to recognize the distinction between these two groups, but he was the first to place importance on the fate of the blastopore as a major distinguishing criterion. Historically, the two groups are distinguished by the following criteria:
1. Embryonic cleavage pattern (that is, how the zygote divides to become a multicellular animal)
2. Fate of the blastopore
3. Origin of mesoderm (the "middle" embryonic tissue layer between ectoderm and endoderm that forms various structures such as muscles and skeleton)
4. Method of coelom formation
5. Type of larva
These developmental features are different in the two groups and can be summarized as follows:
Developmental features of protostomes
1. Cleavage pattern: spiral cleavage
2. Fate of blastopore: becomes the mouth
3. Origin of mesoderm arises from mesentoblast (4d cells)
4. Coelom formation: schizocoely
5. Larval type: trochophore larvaDevelopmental features of deuterostomes
1. Cleavage pattern: radial cleavage
2. Fate of blastopore: becomes the anus
3. Origin of mesoderm: pouches off gut (endoderm)
4. Coelom formation: enterocoely
5. Larval type: dipleurula
Cleavage pattern refers to the process of cell division from one fertilized cell, the zygote, into hundreds of cells, the embryo. In protostomes, the developing zygote undergoes spiral cleavage, a process in which the cells divide at a 45° angle to one another due to a realignment of the mitotic spindle. The realignment of the mitotic spindle causes each cell to di-vide unequally, resulting in a spiral displacement of small cells, the micromeres, that come to sit atop the border between larger cells, the macromeres. Another superphyletic term used to describe animals with spiral cleavage is Spiralia. Spiral cleavage is also called determinate cleavage, because the function of the cells is determined early in the cleavage process. The removal of any cell from the developing embryo will result in abnormal development, and individually removed cells will not develop into complete larvae.
In deuterostomes, the zygote undergoes radial cleavage, a process in which the cells divide at right angles to one an-other. Radial cleavage is also known as indeterminate cleavage, because the fate of the cells is not fixed early in development. The removal of a single cell from a developing embryo will not cause abnormal development, and individually removed cells can develop into complete larvae, producing identical twins, triplets, and so forth.
The fate of the blastopore has classically been used as the defining characteristic of protostomes and deuterostomes. In protostomes, the blastopore develops into the mouth, and the anus develops from an opening later in development. In deuterostomes, the blastopore develops into the anus, and the mouth develops secondarily.
Mesoderm and coelom formation are intimately tied together during development. In protostomes, the mesoderm originates from a pair of cells called mesentoblasts (also called 4d cells) next to the blastopore, which then migrate into the blastocoel, the internal cavity of the embryo, to become various internal structures. In coelomates, the mesentoblasts hollow out to become coeloms, cavities lined by a contractile peritoneum, the myoepithelium. In protostomes, the process of coelom formation is called schizocoely. In deuterostomes, the mesoderm originates from the wall of the archenteron, an early digestive tract formed from endoderm. The archenteron pouches out to form coelomic cavities, in a process called enterocoely.
Protostomia and Deuterostomia are also characterized by different larvae. In most protostomes, the larval type is a trochophore, basically defined by the presence of two rings of multiciliated cells (prototroch and metatroch) surrounding a ciliated zone around the mouth. Most deuterostomes have a dipleurula-type larva, defined by the presence of a field of cilia (monociliated cells) surrounding the mouth.
Contemporary reexamination of Protostomia
For more than a century, biologists have divided the bilateral animals into two main lineages (the diphyletic origin of Bilateria), the most well known of which is the Protostomia/Deuterostomia split. Similar divisions include the Zygoneura/Ambulacralia-Chordonia split proposed by the German invertebrate embryologist Hatschek in 1888, the Hyponeuralia/Epineuralia split proposed by the French zoologist Cuenot in 1940, and a Gastroneuralia/Notoneuralia split proposed by the German zoologist Ulrich in 1951, among others. These divisions often emphasized different developmental and adult features, thereby leading to different names and hypotheses about animal relationships. Although none of these groups have been granted formal taxonomic rank (for example, as a subkingdom or superphylum) by the International Code of Zoological Nomenclature, the names nevertheless remain active in the literature.
Contemporary research on protostome relationships utilizes a host of methods and technologies that were unavailable to biologists in the early twentieth century, such as Grobben. Modern biologists employ electron microscopy, fluorescent microscopy, biochemistry, and a collection of molecular techniques to sequence the genome, trace embryonic development, and gain insight into the origin of various genes and gene clusters, such as Hox genes. Other fields of research, including cladistic analysis and bioinformatics, continue to make important contributions. The latter fields are computer-based technologies that employ algorithms and statistics to handle and analyze large data sets, such as lists of morphological characters and nucleotide sequences. Together with new paleontological discoveries in the fossil realm, these novel techniques and technologies provide modern biologists with a useful way to reexamine the traditional protostome relationships and to develop new hypotheses on animal relationships and evolution.
The evolutionary origin of the Protostomia, and of the groups it includes, remains a major challenge to modern biologists. Although proof of the monophyly of the Protostomia is elusive, many of the phyla are clearly related, and make up clades that some biologists consider monophyletic. For example, in 1997 Aguinaldo et al. proposed the establishment of two clades within the Protostomia based on molecular sequence data: Ecdysozoa (the molting animals, including the Arthropoda, Nematoda, Priapulida, and Tardigrada), and Lophotrochozoa (the ciliated animals, including the Annelida, Echiura, and Sipuncula). Biologists continue to debate these hypotheses and test them with independent biochemical, developmental, molecular, and morphological data.
arrival of new information and a more encompassing examination of all the animal
phyla, the modern view of Protostomia has broadened from that originally
proposed by Grobben, which, at one time or another, included the following
phyla: Brachiopoda, Chaetognatha, Cycliophora, Ectoprocta, Entoprocta, Echiura,
Gastrotricha, Gnathostomulida, Kinorhyncha, Loricifera, Nemertinea, Nematoda,
Nematomorpha, Onychophora, Phoronida, Platyhelminthes, Priapulida, Rotifera,
Sipuncula, and Tardigrada. Many of these phyla contain species that display one
or more developmental characters outlined by Grobben; however, it is rare to
Volume 3. Insects edited by Neil Schlager (Gale Group) Excerpt: We live in the "age of insects." Humans have walked on Earth for only a mere fraction of the 350 million years that insects have crawled, burrowed, jumped, bored, or flown on the planet. Insects are the largest group of animals on Earth, with over 1.5 million species known to science up to now, and represent nearly one-half of all plants and animals. Although scientists do not know how many insect species there are and probably will never know, some researchers believe the number of species may reach 10 to 30 million. Even a "typical"backyard may contain several thousand species of insects, and these populations may number into the millions. It is estimated that there are 200 million insects for every human alive today. Just the total biomass of ants on Earth, representing some 9,000 species, would outweigh that of humans twelve times over. Insect habitats are disappearing faster than we can catalog and classify the insects, and there are not enough trained specialists to identify all the insect specimens housed in the world's museums.
The reproductive prowess of insects is well known. Developing quickly under ideal laboratory conditions, the fruit fly (Drosophila melanogaster) can complete its entire life cycle in about two weeks, producing 25 generations annually. Just two flies would produce 100 flies in the next generation—50 males and 50 females. If these all survived to reproduce, the resulting progeny would number 5,000 flies! Carried out to the 25th generation, there would be 1.192 x 1041 flies, or a ball of flies (1,000 per cubic inch) with a diameter of 96,372,988 mi (155, 097, 290 km), the distance from Earth to the Sun. Fortunately this population explosion is held in check by many factors. Most insects fail to reproduce, suffering the ravages of hungry predators, succumbing to disease and parasites, or starving from lack of suitable food.
Insects are at once entirely familiar, yet completely alien. Their jaws work from side to side, not up and down. Insect eyes, if present, are each unblinking and composed of dozens, hundreds, or even thousands of individual lenses. Insects feel, taste, and smell the world through incredibly sensitive receptors borne on long and elaborate antennae, earlike structureson their legs, or on incredibly responsive feet. Although they lack nostrils or lungs, insects still breathe, thanks to small holes located on the sides of their bodies behind their heads, connected to an internal network of finely branched tubes.
Like other members of the phylum Arthropoda (which includes arachnids, horseshoe crabs, millipedes, centipedes, and crustaceans), insects have ventral nerve cords and tough skeletons on the outside of their bodies. This external skeleton is quite pliable and consists of a series of body divisions and plates joined with flexible hinges that allow for considerable movement.
As our knowledge of insects has increased, their classification has inevitably become more complex. They are now classified in the subphylum Hexapoda, and are characterized by having three body regions (head, thorax, and abdomen) and a three-segmented thorax bearing six legs. The orders Protura, Collembola, and Diplura, formerly considered insects, now make up the class Entognatha. Entognaths have mouthparts recessed into the head capsule, reduced Malpighian tubules (excretory tubes), and reduced or absent compound eyes.
The remaining orders treated in this volume are in the class Insecta. Insects have external mouthparts that are ex-posed from the head capsule, lack muscles in the antennae beyond the first segment, have tarsi that are subdivided into tarsomeres, and females are equipped with ovipositors. The word "insect" is derived from the Latin word insectum, meaning notched, and refers to their body segmentation. The second and third segments of the adult thorax often bear wings, which may obscure its subdivisions.
one of only four classes of animals (with pterosaurs, birds, and bats) to have
achieved true flight, and were the first to take to the air. The evolution of
insect wings was altogether different from that of the wings of other flying
creatures, which developed from modified forelimbs. In-stead, insect wings
evolved from structures present in addition to their legs, not unlike Pegasus,
the winged horse of Greek mythology. Long extinct dragonflies winged their way
through Carboniferous forests some 220 million years ago and had wings measuring
27.6 in (700 mm) or more across. To-day the record for wing width for an insect
belongs to a noctuid moth from
size of insects has allowed them to colonize and exploit innumerable habitats
not available to larger animals. Most species live among the canopies of lush
tropical forests. Some species are permanent residents of towering peaks some
19,685 ft (6,000 m) above sea level. Others live in eternal darkness within the
deep recesses of subterranean caves. Some occupy extreme habitats such as the
fringes of boiling
ecologies of insects are extremely varied, and insects often dominate food webs
in terms of both population size and species richness. Equipped with chewing,
piercing/ sucking mouthparts, or combinations thereof, insects cut, tear, or
imbibe a wide range of foodstuffs, including most plant and animal tissues and
their fluids. Plant-feeding insects attack all vegetative and reproductive
structures, while scavengers plumb the soil and leaf litter for organic matter.
Some species collect plant and animal materials—not to eat, but to feed to their
young or use as mulch to grow fungus as food. Many ants "keep" caterpillars or
aphids as if they were dairy cattle, milking them for fluids rich in
carbohydrates. Predatory species generally kill their prey outright; parasites
and parasitoids feed internally or externally on their hosts over a period of
time or make brief visits to acquire their blood meals.
The concept of "fish" certainly is more steeped in tradition than backed by scientists, despite the fact that countless ichthyologists (i.e., scientists who study fish) have written innumerable pages on the subject. The reality that fishes in the broadest sense have long played important roles in the promotion of industry and commerce, geographic exploration, politics, art, religion, and myth mandates that the definition of fish can vary according to human perspective and sometimes despite science. For example, from a chef's point of view, fishes come in two basic varieties—shellfish and finfish. Scientists eschew such groupings of distantly related creatures. However, lest they be hoisted with their own petards, ichthyologists might tread gently on the many concepts of fish, for they must ac-knowledge science's inability to form an absolute taxonomic definition of "fish" based on biological characteristics that are shared by all fishes and yet not shared with any "nonfish."
Widespread views of the particular characteristics that de-fine fishes, of course, are biased by general familiarity with extant (i.e., living) species and, in particular, with the wide-spread and well-known bony fishes. Thus, the notion of a fish as an aquatic ectothermic vertebrate possessing gills, paired and unpaired fins, and scales usually suffices as a casual definition of fish. Reasonable as this definition may seem, some of these characteristics are shared with other groups of animals that are not considered fishes, while others of them are not common to all fishes. For example, although most fish live in water, some fishes, such as the walking catfish (Clarias batrachus) or African lungfish (Protopterus species) can spend considerable periods out of water. Furthermore, other fishes may spend much briefer, yet highly significant periods out of water, which allow them to feed (e.g., mudskippers, Periophthalmus spp., and the arowanas, Osteoglossum spp.) or flee from predators (e.g., flyingfishes, Exocoetidae).
Similarly, whereas most fishes cannot control their body temperature other than through behavioral mechanisms involving migrations or local movements to and from waters of varying warmth, some lamnids (Lamnidae) and tunas (Thunnus spp.) and the swordfish (Xiphias gladius) can maintain body temperatures that are several degrees higher than the water that surrounds them for significant periods. Certainly, most fishes possess a well-developed vertebral column; however,hagfishes (Myxinidae) lack well-defined vertebrae, and there is disagreement among scientists regarding whether this characteristic exists because the ancestors of these fishes were similar or, antithetically, because vertebrae were "lost" from this lineage through evolutionary modification. In fact, so different are hagfishes from other fishes that Aristotle considered them members of another, illegitimate taxonomic group—worms. Unlike worms, fishes are chordates (phylum Chordata), and they possess skeletal components that form a cranium (i.e., a brain case). This characteristic (as well as many others) distinguishes them from some fishlike chordates, such as the lancelets (Amphioxiformes), but, of course, amphibians, reptiles, birds, and mammals also have a cranium.
Gills cannot be used as an unequivocal characteristic defining fishes, because some amphibians have and use gills for at least a portion of their lives. Furthermore, whereas most fishes obtain oxygen from water through conventional gills, some fishes significantly supplement gill respiration by acquiring oxygen from the water or atmosphere via modified portions of the gills (e.g., the walking catfish) or skin (e.g., the European eel, Anguilla anguilla) or specialized tissues in the mouth (e.g., the North American mudsucker goby, Gillichthys mirabilis), gut (e.g., plecostomuses, Plecostomus species), swim bladder (e.g., the bowfin, Amia calva), or lungs (e.g., the Australian lungfish, Neoceratodus forsteri). Complicating matters still further, some fishes are obligate air breathers and must have access to the atmosphere or they will drown (e.g., the electric eel, Electrophorus electricus and the South American lungfish, Lepidosiren paradoxa).
At first glance, fins seem to define fishes. Several unrelated groups of nonfishes (e.g., lancelets, sea snakes, and some amphibians) possess finlike modifications associated with their tails that facilitate locomotion in water. Furthermore, althoughsome fishes, such as hagfishes and lampreys (Petromyzontidae), lack paired fins, the paired appendages of amphibians, reptiles, birds, and mammals are considered homologous to the paired fins of fishes. Likewise, the scales that cover many common bony fishes are not a universally acceptable distinguishing feature, because numerous unrelated groups of fishes lack scales, for example, the hagfishes, the lampreys, and the North American freshwater catfishes (Ictaluridae). Moreover, those fishes that possess scales may be more or less covered by one of several basic scale types, for example, the placoid scales of sharks, the ganoid scales of gars, and the bony ridge scales of salmon and basses. These differences in the scales of fishes point to the fact that some other aquatic chordates, such as sea snakes, also have scales, even though the outer coverings of reptiles, birds, and mammals are heavily keratinized, whereas those of fishes are not.
Superclass Pisces as a polyphyletic group
Given that no one characteristic distinguishes all fishes from all other organisms, even the most committed ichthyologist must admit that the superclass Pisces (an assemblage that ineludes all fishes) represents an unnatural or polyphyletic group. In fact, given our scientific understanding of fishes as of 2002, the only measure allowing them to stand together as a natural or monophyletic group requires the inclusion of all other craniates (i.e., amphibians, reptiles, birds, and mammals). Most biologists probably would agree that the consideration of all craniates as fishes would be of little scientific value and would betray the longstanding and widespread conception of a fish. In light of this situation, uncompromising cladists returning from a fishing trip for salmon are condemned to telling others of having been "salmoning" rather than "fishing."
General definition of fish
seemingly hopeless conundrum of defining "fish" scientifically, many scientists
and non-scientists probably would agree that a general definition for this
loose group of animals can be established. For these reasonable folks, a fish
can be defined as an ectothermic chordate that lives primarily in water and
possesses a cranium, gills that are useful virtually throughout life, and
appendages (if present) in the form of fins. Those not willing to endorse this
definition might rest easy by considering "fish" as the raison d'etre for
edited by Neil Schlager (Gale Group) Excerpt: Almost everyone recognizes a fish,
a bird, or a mammal, even a reptile. But what about an amphibian? Most people
recognize frogs and toads as amphibians, but these animals are not the only
Amphibia, a class of vertebrates (back-boned animals). There are three living
groups of amphibians. The most generalized are salamanders, order Caudata (=
with tail), having a cylindrical body, long tail, distinct head and neck, and
usually well-developed limbs of approximately equal length. Most salamanders are
terrestrial, but some are aquatic, a few are burrowers, and some others are
arboreal. Frogs, order Anura (= without tail), have a robust body continuous
with the head, no tail, and long hind limbs. Most frogs are terrestrial or
arboreal, but many are aquatic, and a few are burrowers. The third group
contains the caecilians, order Gymnophiona, also called Apoda (= without foot).
These limbless amphibians superficially resemble earthworms and have blunt heads
and tails, and their elongate bodies are encircled by grooves (annuli). A few
caecilians are aquatic, but most burrow in soil in tropical regions of the
In some ways amphibians are intermediate between the fully aquatic fishes and the terrestrial amniotes (reptiles, birds, and mammals), but they are not simply transitional in their morphology, life history, ecology, and behavior. During their nearly 350 million years of evolution, amphibians have undergone a remarkable adaptive radiation, and the living groups exhibit a greater diversity of life history than any other group of vertebrates.
Basically, amphibians can be defined as quadrupedal vertebrates (four-legged, or tetrapods) with a skull having two occipital condyles (articulating surfaces with the first element of the vertebral column). The attachment of the pelvic girdle to the vertebral column incorporates only one sacral vertebra. In anurans (frogs and toads), the postsacral vertebrae are fused into a rodlike structure, the urostyle (coccyx), and a tail is absent. Caecilians and some salamanders lack limbs and girdles, whereas in anurans the hind limbs are elongated and modified for jumping. The skin is glandular and contains both mucous and poison glands but lacks external structures such as scales, feathers, or hair, characteristic of other groups of tetrapods. The heart has three chambers, two atria and one ventricle,which may be partially divided. The aortic arches are symmetrical. Typically, amphibians have two lungs, but the lungs may be reduced or absent in some salamanders, and the left lung is proportionately small in most caecilians (as it is in snakes). Some features are unique to amphibians, all of which have teeth that consist of a pedicel and a crown, and specialized papillae for sound reception in the inner ear. Amphibians are ectotherms (cold-blooded). They are unable to regulate their body temperatures physiologically, as do birds and mammals; therefore, their body temperatures approximate those of the immediate environment, especially the substrate.
The life histories of amphibians are highly diverse. The classic amphibian life history of aquatic eggs and larvae is only one of many modes of reproduction, which include direct development of terrestrial eggs (no aquatic larval stage) and live birth. The eggs of amphibians lack a shell and the embryonic membranes (e.g., amnion, allantois, and chorion) of reptiles, birds, and mammals. Instead, amphibian eggs are protected only by mucoid capsules that are highly permeable; thus, amphibian eggs must develop in moist situations.
Phylogenetic relationships and classification
The living groups of amphibians are most closely allied with diverse fossils, the basal tetrapod vertebrates commonly placed in the class Amphibia. The phylogenetic relationships among these groups of fossils is equivocal. Based on morphological and molecular evidence, salamanders and anurans form a monophyletic group (i.e., have a common ancestor) and together are referred to as batrachians. Batrachians and caecilians form another monophyletic group, the lissamphibians.
Classification reflects biologists' knowledge of the relationships of groups of organisms. Consequently, as new characteristics, both morphological and molecular, as well as behavioral and developmental, are discovered and analyzed, the classification changes. New evidence may reveal that a group of species or genera that were once believed to be members of one family are actually more closely related to another group or are not related to the family with which they formerly were associated. For example, salamanders in the families Dicamptodontidae and Rhyacotritonidae formerly were placed in the Ambystomatidae. Likewise, African treefrogs now recognized as the family Hyperoliidae formerly were in the Rhacophoridae, and frogs formerly recognized as the family Pseudidae are now assigned to a subfamily of Hylidae.
(the study of evolution and classification of organisms) is a dynamic field,
and the relationships of many groups are still being unraveled. Depending on
which kinds of evidence are used, the results may differ and different
classifications may be proposed. The relationships of some groups of living
amphibians have not been resolved with a high level of confidence. For example,
a group of frogs endemic to
distributions of the families of amphibians reflect the history of Earth,
especially from the time of the breakup of the supercontinent Pangaea, beginning
about 190 million years ago. The early fragmentation resulted in two major land
masses: Laurasia, consisting of what is now
fossil caecilian is known from the Jurassic of North America, these amphibians
now all live in regions that were part of Gondwana. Two families are restricted
to the Indian subcontinent (one in adjacent southeastern
evolved in Laurasia. One family is restricted to
biogeography of anurans is somewhat more complicated. One early lineage
containing the living Ascaphidae in
As a group, amphibians are distributed throughout the world, except for polar regions, most oceanic islands, and some desert regions. However, the patterns of distribution differ among the three living groups of amphibians. Anurans occur throughout the world but are most diverse in the tropics; salamanders are most diverse in the northern continents; and caecilians are restricted to the tropics.
except for the
caecilians are pantropical, they are absent in
salamanders live in the Northern Hemisphere; they are absent in the
Australo-Papuan and Ethiopian regions. At the family level, the greatest
diversity is in the Nearctic region, where all families (except Hynobiidae)
occur, and five families (Ambystomatidae, Amphiumidae, Dicamptodontidae,
Rhyacotritonidae, and Sirenidae) are endemic. Cryptobranchidae and Proteidae
are represented by one genus each in the Nearctic and Palearctic regions.
Salamandridae is the most widespread family of salamanders with nine genera in
the Palearctic region, and two genera in the Nearctic region. Hynobiidae is the
only family of salamanders restricted to the Palearctic region. By far, the
largest family of salamanders is Plethodontidae with 25 genera in the Nearctic;
one of these (Hydromantes) is shared with
of the 28 families of anurans occur in both the Old and New Worlds. Bufonidae is
global in its distribution, except for
exception of Pelobatidae in the Nearctic and Palearctic regions, all other
families of anurans are restricted to the
Myobatrachidae are endemic to the Australo-Papuan region. The Ethiopian region
has six endemic families of anurans: Arthroleptidae, Heleophrynidae,
Hemisotidae, Hyperoliidae (
Neotropical region has the world's greatest diversity of anurans. In addition to
many genera and species of Bufonidae, Hylidae, and Microhylidae, there are
seven endemic families: Allophrynidae, Brachycephalidae Centrolenidae,
Dendrobatidae, Leptodactylidae, Rhinodermatidae, and Rhinophrynidae. Four of
these (Allophrynidae, Brachycephalidae, Rhinodermatidae, and Rhinophrynidae)
contain a total of only eight species, but Centrolenidae and Dendrobatidae have
a total of more than 300 species, and Leptodactylidae contains more than 1,000
species, of which Eleutherodactylus is the most speciose and widespread. Other
families in the Neotropical region are Pipidae (shared with
Amphibians in the ecosystem.
Although amphibians are generally restricted to moist environments, such as humid forests, marshes, ponds, and streams, many species venture far from free-standing water and inhabit trees, rocky cliffs, and soil under the surface of the ground. In such diverse habitats, amphibians feed on a great variety of smaller organisms, principally invertebrates, of which insects are the most common in the diets of anurans and salamanders. However, their diets also include earth-worms (especially in caecilians), small snails, spiders, and other small invertebrates. Body size plays an important role in prey selection. Some aquatic salamanders feed on tadpoles, and a few larger aquatic salamanders feed on fishes; the eel-like aquatic amphiumas feed almost exclusively on crayfish. Many species of frogs are less than 1 in (25 mm) in head-body length, and their diets are restricted to small insects and spiders. In tropical forests, many of these small frogs specialize on ants and termites, both of which are abundant. Large frogs with wide gapes tend to eat larger prey, which may include other frogs, lizards and small snakes, birds, and mammals. Tadpoles feed primarily on decaying vegetation, algae, and plankton in ponds and streams.
The dietary habits of amphibians are important in the ecosystem because as adults they consume vast quantities of insects and thus help to maintain a balance in the ecosystem. Areas where local anurans have been eliminated have witnessed large population increases in some kinds of insects, and mountain streams that once were relatively free of algae can become choked with algae when algal-feeding tadpoles disappear.
Because of their abundance and relative ease of capture, amphibians are included in the diets of a great variety of animals, especially many small mammals, birds, and many kinds of snakes. Wading birds feast on tadpoles and metamorphosing frogs in shallow ponds. A few snakes specialize on salamanders, and many kinds of snakes in the tropics feed almost exclusively on frogs. Small salamanders and frogs also fall prey to spiders. Even subterranean caecilians cannot escape predation by some snakes, especially coral snakes of the genus Micrurus.
amphibians are a significant part of the food web in most terrestrial ecosystems
on the planet. In the late 1980s, biologists realized that populations of
amphibians were declining in many parts of the world. Gradual, and especially
precipitous, declines result not only in the potential loss of species of
amphibians, but have a significant impact on the populations of their prey and
those of their predators and animals farther up the food chain. The long-term
effects of these declines have yet to be determined.
Volume 7. Reptiles edited by Neil Schlager (Gale Group) Excerpt: The difference between amphibians and reptiles is that reptiles exhibit a suite of characteristics understandable as adaptations to life on land at increasing distance from water. Although many species of amphibians live on land in adult-hood, most have an aquatic larval stage, and few can exist for long without moisture even during their terrestrial stages of life. Amphibians are tied to water—most species are not found more than a few meters from water or from moist soil, humus, or vegetation. Reptiles of many species are relatively liberated from water and can inhabit both mesic (moist) and xeric (dry) environments. Reptiles need water for various physiological processes, as do all living things, but some reptiles can obtain the water they need from the foods they eat and through conservative metabolic processes without drinking or by drinking only infrequently. Understanding the nature of reptiles requires focus on their techniques for maintaining favorable water balance in habitats where water may not be readily available and where moist microniches may be uncommon.
Most reptiles have horny skin, almost always cornified as scales or larger structures called scutes or plates. Such integuments resist osmotic movement of water from body compartments or tissues into the surrounding air or soil, thus minimizing desiccation. There are times in the lives of snakes and lizards when their skin becomes permeable to water, as when the animals are preparing to shed their old skin. During such times they seek out favorable hiding places that protect them not only from predators but also from water loss. The combination of integumentary impermeability (most of the time) and innate preferences for favorable microclimates during vulnerable periods allows reptiles to retain body water rather than to lose it to arid surroundings. Some reptiles are known to drink water that condenses on their scales when they reside in cool burrows.
Added to the mechanisms for retaining body water is an excretory system that is considerably advanced over those in fishes and many amphibians. The kidneys are integral components of the circulatory system. They allow constant, efficient filtration of blood. Most aquatic organisms excretenitrogenous waste as ammonia. Ammonia readily diffuses across skin or gills, provided plenty of water is present, but is not efficiently excreted by the kidneys. Ammonia is highly toxic, and animals cannot survive if this substance accumulates in their bodies. Terrestrial organisms excrete nitrogenous waste in the form of urea or uric acid, which are less toxic and which require less water than does excretion of ammonia. Urea is the main nitrogenous waste in terrestrial amphibians, whereas uric acid (which requires very little water) is the main nitrogenous effluent in reptiles. Finally, some desert-dwelling reptiles have a remarkable ability to tolerate high plasma urea concentrations during drought. This characteristic allows the animals to minimize water loss that would be coincident with excretion. Rather than being excreted, nitrogenous waste is simply retained as urea, and water is con-served. When a rainfall finally occurs, reptiles (e.g., the desert tortoise Gopherus agassizii) drink copiously, eliminate wastes stored in the bladder, and begin filtering urea from the plasma. Within days their systems return to normal, and the tortoises store a large volume of freshwater in their bladders to deal with the next drought.
Feeding in a water medium among vertebrates can take several forms ranging from detritus feeding (ingestion of decaying organic matter on the substrate) to neuston feeding (ingestion of tiny organisms residing in the surface film). Probably the most common mechanism of obtaining food is suction feeding, whereby the predator creates a current by sucking water into the expanded buccal cavity and out through gills, causing prey to be captured in the mouth. Most fish rely on suction feeding, and this mechanism contributes to the effectiveness of detritivores, neustonivores, and aquatic predators. As a consequence, most fish have relatively weak mouths and low bite strength. There are exceptions, such as sharks, but the general rule is that fish depend on suction more than on biting, a circumstance that works effectively because of the liquid nature of the water medium and the associated friction arising between the medium and objects suspended in it. Aquatic amphibians also use suction feeding, although some species have lingual and jaw prehension, particularly during terrestrial stages. The transition to land dwelling among most reptiles has necessitated a revolution in oral structures and kinematics to cope with the less dense medium of air. Because suction feeding does not work effectively in air, jaw prehension with consequent increases in bite strength has been emphasized in the evolution of most reptiles. Jaw prehension involves increased number and volume of the jaw-suspending muscles and increased surface area of muscle origins. Associated with this development was the appearance of temporal openings in the dermal bone surrounding the brain, because these openings allowed some of the jaw-suspending muscles to escape from the constraints of the dermal-chondral fossae and to attach at origin sites on the lateral and dorsal surfaces of the skull.
The number and position of temporal openings have been used to classify reptiles into taxonomic groups, and the high-lights of this classification system are reviewed here. Reptile skulls lacking temporal vacuities are said to be anapsid (with-out openings). This group includes the fossil order Cotylosauria, also called stem reptiles because of their ancestral position to all higher reptiles and hence to birds and mammals. The turtles, order Testudines, also are anapsid. Synapsid skulls have a single temporal opening on each side. The opening is positioned relatively low along the lateral surface of the skull, within the squamosal and postorbital bones. All synapsid reptiles (orders Pelycosauria, Therapsida, and Mesosauria) are extinct, but they are of great interest because of their ancestral position relative to the mammals. The parapsid condition also has a single vacuity on each side, but it is located rather high on the dorsolateral surface of the skull, within the supratemporal and postfrontal bones. Extinct, fishlike members of the order Ichthyosauria constitute the single order of parapsid reptiles, but these animals were probably closely related to euryapsid reptiles that had a single vacuity in much the same position except that it also invaded the dorsal aspects of the squamosal and postorbital bones. Orders of euryapsids were Placodontia and Sauropterygia, both marine and extinct, in the Triassic and Cretaceous periods, respectively. The diapsid condition is characterized by two temporal vacuities on each side of the skull. Major orders include Thecodontia (small crocodilian-like reptiles ancestral to birds and to all of the archosaurs), Crocodylia, Saurischia (dinosaurs with ordinary reptile-type hips), Ornithischia (dinosaurs with bird-type hips), Pterosauria (flying reptiles), Squamata (lizards, snakes, and several extinct groups), Eosuchia (extinct transitional forms that led to squamates), and Rhynchocephalia (mostly extinct, lizard-like diapsids with one surviving lineage, the tuatara [Sphenodon punctatus] on islands associated with New Zealand; S. punctatus may be a superspecies containing two or more separable species).
The order Testudines, which contains all living and extinct turtles, has traditionally been grouped with the primitive cotylosaurs because of common possession of the anapsid condition. Most herpetologists and paleontologists have agreed on this matter for many years. Molecular geneticists, however, have found evidence that turtles may actually be closely related to diapsid reptiles. This finding suggests that the anapsid condition of turtles may be secondary. That is, turtles may have evolved from ancestors that possessed two temporal vacuities on each side of their skulls, but in the course of evolution, turtles lost these openings. Essentially the same idea was proposed early in the twentieth century, not on the basis of genetic evidence but on the basis of a paleontological scenario involving a series of extinct but turtle-like diapsid fossils. Few at that time could accept the possibility that temporal vacuities once evolved would ever be abandoned, so this notion was dismissed and has resided in scientific limbo ever since. It has been revived on the strength of genetic data, and this much derided "preposterous idea" may become accepted.
It appears as if there is a contradiction associated with the anapsid status of turtles. Whereas some species are suction feeders with relatively weak mouths, others, such as snapping turtles, have profound bite strength. How is this strength produced, given the absence of temporal openings that would al-low large jaw-suspending muscles to anchor (originate) on the dorsal surface of the skull? It turns out that many species of turtles have an analogous adaptation in which sections of dermal bone on the side and back of the skull have become emarginated or notched. Temporal openings are holes surrounded by bone. Emarginations are missing sections of the edges of the flat bones that form the ventral or pleural borders of the skull. With substantial sections of these bones missing, jaw-suspending muscles have the same opportunity to escape from the dermal-chondral fossae as is made possible by vacuities. Although turtles are, strictly speaking, anapsid, some have taken an alternative pathway that leads to the bite strength necessary for effective jaw prehension of substantial prey or for tearing vegetation. If the anapsid condition is secondary, turtles have substituted an analogous trait that accomplished much the same biophysical effect as did the former temporal vacuities.
The earliest reptile fossils known are from the Upper Carboniferous period, approximately 270 million years ago, but by this time several of the reptilian orders were already in evidence, including both anapsid cotylosaurs and synapsid pelycosaurs. This finding implies that reptile evolution began much earlier. Another implication is that temporal vacuities (empty spaces) and emarginations (notches), although widely distributed in reptiles, are not defining characteristics of this class of vertebrates, because several groups do not have them. The earliest defining characteristics may never be known unless some very early fossils in good condition are found. It is likely that a desiccation-resistant integument was present. An-other area on which to focus is the egg and the reproductive process. The egg is macrolecithal (contains much yolk) and is surrounded by a hard shell in turtles, crocodilians, and geckos and a soft or parchment-like shell in the other squamates. In either case, a shelled egg requires that fertilization occur before shell formation. This means that fertilization must take place within the female's body (i.e., in her oviducts) rather than externally as is typical of fishes and amphibians. Consequently, most male reptiles possess copulatory organs that deposit sperm into the cloaca of the female. From the cloaca the sperm cells migrate up the oviduct guided by chemical stimuli. Male turtles and crocodilians have a single penis homologous to the penis of mammals. This organ develops during embryogenesis from the medial aspect of the embryonic cloaca. Male lizards and snakes have paired hemipenes, which develop during embryogenesis from the right and left lateral aspects of the embryonic cloaca. Some male snakes have bifurcated hemipenes, so the males appear to have four copulatory organs. Thus internal fertilization is the rule among extant reptiles. Even tuatara, the males of which lack copulatory organs, transfer sperm in the manner of most birds with a so-called cloacal kiss involving apposition of male and female cloacae and then forceful expulsion of seminal fluid directly into the female's cloaca. Internal fertilization is necessary because of shell formation around eggs. Many reptiles live far from standing or running water, thus external fertilization in the manner of most fishes or amphibians would be associated with risk of desiccating both sperm and eggs.
The oviducts of some female reptiles are capable of storing sperm in viable condition for months or even years. In some turtles and snakes, fertilization can occur three years after insemination. Theoretically, a female need not mate each year, but she might nevertheless produce young each year using sperm stored from an earlier copulation. Although this interesting possibility has been known from observation of captive reptiles for approximately five decades, we still do not know whether or how often female reptiles use it under natural conditions. Another curiosity of reptile reproduction is that the females of some species of lizards and snakes are capable of reproducing parthenogenetically, even though re-production in these species normally occurs sexually. (These species should not be confused with others that only repro-duce parthenogenetically. This is not a widespread mode of reproduction in reptiles, but it is known to occur in several species of lizards and at least one snake.) Facultative parthenogenesis has only recently been discovered among captive rep-tiles, and there is as yet no information on whether it occurs in nature.
Macrolecithal eggs allow embryos to complete development within the egg or within the mother in the case of viviparity, such that the neonate is essentially a miniature version of its parents rather than a larva that must complete development during an initial period of posthatching life, as is common among amphibians. The reptilian embryo lies at the top of the large supply of yolk, and cell division does not involve the yolk, which becomes an extra embryonic source of nourishment for the growing embryo. A disk called the vitelline plexus surrounds the embryo and is the source of the three membranes (chorion, amnion, and allantois) that form a soft "shell" within the outer shell of the reptilian egg. Together these structures defend the water balance of the developing embryo and store waste products. Although reptile eggs absorb water from the substrate in which they are de-posited, these eggs do not have to be immersed in water as is required for the eggs of most amphibians. Immersion of most reptile eggs results in suffocation of the embryos. Female reptiles deposit their eggs in carefully selected terrestrial sites that provide adequate soil moisture and protect the eggs from extremes of temperature.
Some species have another strategy for protecting embryos from abiotic and biotic exigencies. These reptiles retain the embryos and incubate them within the maternal body. The mother's thermoregulatory and osmoregulatory behaviors contribute to the embryos' welfare and to the mother's welfare. The mother's predator-avoidance behaviors can enhance the fitness of embryos exposed to greater predation elsewhere. In view of these potential advantages, which in some habitats might be considerable, it is not surprising that live-bearing has evolved many times in reptiles, although it is quite rare in amphibians. All crocodilians, turtles, and tuatara are egg layers. At least 19% of lizard species and 20% of snakes are live-bearers. Cladistic studies have shown that viviparity has evolved independently many times within squamates, in at least 45 lineages of lizards and 35 lineages of snakes. It also appears that viviparity is an irreversible trait and that once viviparity evolves, oviparous descendants rarely occur. The term embryo retention is used for species in which females retain embryos until very near the completion of embryogenesis when shells are added. The eggs are deposited and then hatch within 72–96 hours. Examples include the North American smooth green snake Liochlorophis vernalis, and the European sand lizard Lacerta agilis. Most important to understand is that the embryos are lecithinotrophic (nourishment of the embryos comes entirely from the yolk) with no additional postovulatory contribution from the mother. The mother, however, may play a role in gas exchange of the embryos. This process can involve proliferation of maternal capillaries in the vicinity of the embryos, a form of rudimentary placentation. Some species that give birth to live young also have lecithinotrophic embryos that undergo rudimentary placentation. Some embryo-retaining species eventually add a shell to their eggs and oviposit them within a few days of hatching. Others never add a shell, and the young are simply born alive, although they need to extricate themselves from the extraembryonic membranes that surround them. Many herpetologists prefer to abandon the term ovoviviparous because this word connotes that shelled eggs hatch in the maternal oviduct. No species is known in which this occurs. Accordingly, the term viviparous is used for all live-bearers, and herpetologists recognize that considerable variation exists in the degree to which viviparous embryos are matrotrophic (sup-ported by maternal resources through a placenta).
Although females of oviparous species deposit their eggs in sheltered positions, the vagaries of climate can result in relative cooling or heating of oviposition sites with associated changes in moisture. This realization has led to considerable research on the effects of these abiotic factors on embryonic development. It is now known that within the range of 68–90°F (20–32°C), incubation time can vary as much as five-fold, and that neonatal viability is inversely related to incubation time. Hatchlings from rapidly developing embryos at high temperatures perform poorly on tests of speed and en-durance relative to hatchlings from slower-developing embryos at lower temperatures. The slower-developing embryos typically give rise to larger hatchlings than do their rapidlydeveloping counterparts. In the context of this work, it was found that the sex ratio of hatchling turtles varied depending on incubation temperature. In several species of tortoise (Gopherus and Testudo), for example, almost all embryos became males at low incubation temperatures (77–86°F [25-30°C]), and most became females between 88°F and 93°F (31–34°C). Temperature-dependent sex determination (TSD) is known to be widespread, occurring in 12 families of turtles, all crocodilians, the tuatara, and in at least three families of lizards. However, the effect of temperature differs in the various groups. Most turtles exhibit the pattern described, whereas most crocodilians and lizards exhibit the opposite pattern, females being produced at low incubation temperatures and males at higher ones. In a few crocodilians, turtles, and lizards females are produced at high and low incubation temperatures and males at intermediate temperatures. It is possible that some viviparous species experience TSD, in which case the thermoregulatory behavior of the mother would deter-mine the sex of the embryos, but this phenomenon has not been observed.
The effect of the discovery of TSD has been enormous. Almost all developmental biologists previously believed that sex in higher vertebrates was genetically determined. This phenomenon has important implications for the management of threatened or endangered populations, especially if the pro-gram contains a captive propagation component. Unless care is taken to incubate eggs at a variety of temperatures, the pro-gram could end up with a strongly biased sex ratio. Reflection on the effects of global warming on reptiles exhibiting TSD generates the worry that extinction could be brought about from widely skewed sex ratios.
Diversity of reptiles
Reptiles range in body form from crocodilians to squamates, tuatara, and turtles. This diversity borders on trivial, however, in comparison with the range of forms and lifestyles that existed during the Jurassic and Cretaceous periods. This point can be further appreciated by considering locomotion among lizards with well-developed legs. Although some species are capable of quick movement, the gait of all lizards is basically the same as that of salamanders. The legs extend from the sides and must support the body through right angles, greatly limiting body mass and speed. Within the con-text of these constraints, lizards do quite well, but their locomotion remains relatively primitive. Truly advanced locomotion, with the legs directly under the body, occurs among mammals, but this pattern of limb suspension evolved in dinosaurs and was clearly a part of their long period of success. All extant reptiles are ectotherms, deriving their body heat from radiation, conduction, or convection, whereas mammals and birds are endotherms, producing body heat by energy-consuming metabolic activity. Thus we see the primitive condition in the reptiles and the advanced condition in the birds and mammals. There is now good reason to believe that at least some dinosaurs were endotherms. Accordingly, it is important to keep in mind that the diversity of extant reptiles is but a fraction of the diversity exhibited by this class of vertebrates during earlier phases of its natural history.
pattern of the tetrapod limbs of amphibians is preserved in reptiles: a single
proximal bone is followed distally by paired bones. In the fore limb is the
humerus followed by the radius and ulna. In the hind limb is the femur followed
by the tibia and fibula. The wrist and hand are formed from the carpal and
metacarpal bones, and the ankle and foot are formed from the tarsals and
metatarsals, five or fewer digits bearing horny claws distal to both wrist and
ankle. Reptile orders show enormous variation in the precise form and
arrangement of these basic elements and in their behavioral deployment. In
squamates these elements are abandoned in favor of serpentine locomotion, which
requires an elongate body and therefore an increased number of vertebrae, more
than 400 in some snakes. Serpentine locomotion depends on friction between the
animal and the substrate, which in some animals is accomplished by pressing the
posterior edges of the belly scales against stationary objects so that
The brain and spinal cord exhibit several advanced characteristics in reptiles relative to amphibians, including larger size and greater definition of structural divisions and greater development of the cerebral cortex. Neural connections between the olfactory bulbs, the corpus striatum, and several other subcortical structures have become clearly established in reptiles, and these connections have been conserved in subsequent evolution such that they are present in mammals, including humans. This set of connections is sometimes referred to as the "reptilian brain" or "R-complex" and is thought to represent a neural circuit necessary for the mediation of basic functions such as predation and mating as well as the affective concomitants associated with social behaviors ranging from cooperation to aggression. In the study of mammals, we speak of the regulation of emotion by components of the reptilian brain. Herpetologists are generally reluctant to speak of emotion in their animals, but they have no difficulty recognizing the existence of the neural circuit in question and in understanding that it contributes to social and reproductive activities. Whether this contribution is limited to the organization of motor patterns or whether emotion also is involved remains an open question.
Sensory structures of reptiles exhibit variations in size and complexity that are roughly correlated with ecological variation and phylogeny. For example, lizards considered to be primitive, such as those of the family Chamaeleonidae, are primarily visually guided in the context of predation as well as in the contexts of social and reproductive behavior. This reliance on vision is reflected in the wonderful mobility of the eyes, the size of the optic lobes, and in the brilliant color patterns in the family. The phenomenon of "excited coloration" (color changes reflecting emotional or motivational states) involves socially important signals that can only be appreciated with vision. More advanced lizards, such as those in the family Varanidae, place greater emphasis on their nasal and vomeronasal chemosensory systems. Associated with this characteristic is a shift in the morphology and deployment of the tongue, which in varanids is used mainly to pick up nonvolatile molecules and to convey them to the vomeronasal organs. There is an associated shift from insectivory to carnivory. In snakes, which may be derived from a varanid-like ancestor, these shifts have been carried to an even greater extreme.
Audition presents an interesting problem in reptiles. Snakes and some lizards have no external ear, although the middle and inner ears are present. In species with a distinct external auditory meatus, there is little doubt about the existence of a sense of hearing, although it is generally thought that only sounds of low frequency are detected. In species lacking an external ear, seismic sounds are probably conducted by the appendicular and cranial skeletons to the inner ear. It has been suggested, however, that the lungs might respond to airborne sounds and transmit them to the inner ear via the pharynx and eustachian tube. Although no reptiles are known for having beautiful voices, many generate sounds. For ex-ample, male alligators bellow, and this sound undoubtedly serves social functions. Many snakes hiss, some growl, and a fair number issue sounds with their tails either with a rattle or by lashing the tail against the substrate. Such sounds are generally aimed at predators or other heterospecific intruders, and herpetologists have believed that the issuing organ-ism was deaf to its own sound, unless the sound had a seismic component. Perhaps this view can be altered if the concept of pneumatic reception of airborne sound is corroborated.
Cutaneous sense organs are common, including those sensitive to pain, temperature, pressure, and stretching of the skin. Although pain and temperature receptors are best known on the heads of reptiles, these receptors are not confined there. The mechanoreceptors that detect touch, pressure, and stretch are present over the body, especially within the hinges of scales. Receptors that detect infrared radiation (heat) are also of dermal and epidermal origin. In boas and pythons, these receptors are associated with the lips. In pitvipers such as rattlesnakes, a membrane containing heat receptors is stretched across the inside of each pit approximately 0.04—0.08 in (1—2 mm) below the external meatus. The geometry of the bilateral pits is such that their receptive fields overlap, allowing stereoscopic infrared detection. The nerves of the pits project to the same brain areas as do the eyes, giving rise toimages containing elements from the visible part of the spectrum as well as the infrared part. When a pitviper is in the process of striking a mouse, the snake's mouth is wide open with fangs erect, so that the pits and eyes are oriented up rather than straight ahead toward the prey. It turns out that in the roof of the mouth near the fangs are additional infrared sensitive receptors that appear to take over guidance of the strike during these final moments.
Reptiles also possess proprioceptors associated with muscles, tendons, ligaments, and joints. Proprioceptors report the positions of body components to the brain, allowing the brain to orchestrate posture and movement. Another class of internal receptor contains taste buds, which are located in the lining of the mouth and on the tongue. In reptiles with slender, forked tongues specialized for conveying nonvolatile chemicals to the vomeronasal organs, lingual taste buds are generally absent, but taste buds may be present elsewhere in the mouth.
With a few notable exceptions, the teeth of extant reptiles are unspecialized; that is, most teeth look alike, and the dentition is called homodont (Latin for "alike teeth"). The teeth may vary considerably in size along the length of the tooth-bearing bones, especially in snakes, because the teeth are deciduous and are replaced regularly. This type of dentition is called polyodont. Teeth are present on the bones of the upper and lower jaw and on other bones forming the roof of the mouth (palatine and pterygoid). If teeth are ankylosed (cemented by calcification) to the inside of jawbones, the dentition is pleurodont. This is the condition of all snakes and most lizards. If the teeth are ankylosed to a bony ridge along the jawbones, as in some lizards, the dentition is acrodont. Crocodilian teeth are situated in sockets, as are the teeth of mammals, and this dentition is called thecodont. The most spectacular type of tooth specialization in extant reptiles involves the fangs of venomous snakes. These fangs are hollow, elongated teeth on each side of the front of the upper jaw, although some species have solid, grooved fangs on each side of the rear of the upper jaw. In front-fanged snakes, venom is forcefully injected through the fangs and exits into the prey through slitlike openings on the lower anterior face of each fang. In rear-fanged snakes, venom runs under little pressure along the grooves and enters prey as the rear fangs successively embed themselves into prey during swallowing. Among the front-fanged species are those with folding fangs that are normally held parallel to the roof of the mouth and rotated down into position as needed. Other front-fanged snakes have less mobility associated with their fangs, which are therefore always in the biting position. The fangs typically are much longer in species with folding fangs than in species with fixed front fangs. With the exception of fangs, most teeth in extant reptiles are used to grip prey, although some lizards have specialized, blunt teeth that crush snail shells. Some extinct reptiles had far more specialized tooth patterns than do the surviving groups.
All reptiles possess salivary glands that lubricate food and begin the process of digestion. Saliva also cleans the teeth by digesting pieces of organic matter that might adhere to the teeth or be stuck between adjacent teeth. The venom that has evolved in snakes undoubtedly arose from salivary glands, and it has retained its original digestive function. Venom contains elements that immobilize and kill prey, and it facilitates digestion. It has been conclusively demonstrated in force-feeding experiments in which rattlesnakes fed envenomated mice completed the digestion process significantly quicker than did conspecifics fed identical euthanized mice that had not been envenomated. Similar studies have been completed with comparable results for a variety of species, including rear-fanged snakes. In some rear-fanged snakes, venom is apparently used only for digestion and not for subduing prey or for defense. In the Mexican beaded lizard (Heloderma horridum) and the Gila monster (H. suspectum), the only venomous lizards, venom is apparently used strictly for defense and not for acquisition or digestion of prey.
In some snakes and lizards, very long periods of time can occur between successive meals, and the reptiles exhibit an interesting form of physiological economy by down-regulating their digestive machinery. This process saves energy, because maintaining functional digestive tissue in the absence of food would require considerable caloric costs. Reptiles retain this down-regulated condition until the next meal has been secured, at which time the gut is up-regulated.
Gas exchange occurs through lungs. Most snakes have only one lung (on the left). The heart has three chambers, two atria and one ventricle, except in crocodilians, in which a second ventricle is present, producing a four-chambered heart much like that of mammals. Even in reptiles with a three-chambered heart, a septum exists within the ventricle and minimizes mixing of oxygenated and nonoxygenated blood. Researchers have studied the physiological mechanisms associated with exhaustive locomotion and have found interesting parallels between reptiles and mammals in the rapidity of recovery from exhaustion. A major difference, however, is that mammals exhibit a so-called exercise effect (exercise-induced ability to mobilize greater levels of oxygen and, hence, to work harder than was possible before exercise), whereas no reptile has yet been shown to do this.
of reptiles continue to be discovered. This is especially true of lizards. Hence
the numbers that follow are approximations subject to change. We currently
recognize 285 species of turtles, 23 crocodilians, two tuatara, 4,450 lizards,
and 2,900 snakes. One of the authors of this chapter (H. M. S.) has named
approximately 300 species in his career and is working on projects that will
almost certainly add species to the list. In countries such as the
Many of the
same factors responsible for amphibian de-clines have been insidiously working
their decimating effects on reptiles. At the heart of the problem is the human
population, now much more than six billion, and a drastically uneven
distribution of resources. Many people living in areas of high reptile diversity
are unable to eke out a living and are therefore tempted to exploit their native
fauna, legally or illegally, and to engage in other economic activities that
eventually have negative repercussions on the fauna. Hunting of reptiles occurs
for local consumption, sale of hides or shells, sale of live animals to the pet
trade, and sale of meat or other body parts as exotic food or medicines.
situation developed in connection with hides of various reptiles, including
crocodilians and several large lizards and snakes. In the case of crocodilians,
management programs aimed at providing sustainable yields were developed in
several countries, and these measures proved successful, so much so that the
species involved recovered from endangered status. This experience indicates
that the conservation strategy of management for sustainable yield can work if
it is carefully implemented on the basis of good ecological and demographic
data and if the harvest is carefully monitored. Enthusiastic participation of
local people is an important element of the success of such programs as they
have been carried out in
Volume 8-11. —
Birds IV: edited by Donna Olendorf (Gale Group) Though too bulky for field
use this four volume summary of birds of the world will be a welcome addition to
any ornithologist, amateur or professional. Highly recommended.
Volume 8-11. — Birds I-IV; Birds I: Birds II: Birds III: Birds IV: edited by Donna Olendorf (Gale Group) Though too bulky for field use this four volume summary of birds of the world will be a welcome addition to any ornithologist, amateur or professional. Highly recommended.
Excerpt: Everyone recognizes birds. They have feathers, wings, two legs, and a bill. Less uniquely, they have a backbone, are warm-blooded, and lay eggs. All but a few birds can fly. Birds have much in common with reptiles, from which they have evolved. They share several skeletal characteristics, nucleated red blood cells, and their young develop in cleidoic eggs. The main difference is feathers, which are modified scales. Not only do feathers allow flight, they are insulated, more so than mammalian hair, enabling birds to maintain steady internal temperatures and stay active even in extreme climates. The acquisition of flight and homeothermia has influenced the evolution of other anatomical and physiological changes in birds and led to increased cerebral and sensory development. It has freed them to travel the globe, colonizing most environments and diversifying to fill many ecological niches. Consequently, it is not surprising that birds are the most successful of the vertebrates, outnumbering the number of mammal species twofold.
Evolution and systematics
record of birds is patchy and their evolutionary history is poorly known. The
first feathered animal, Archaeopteryx, has been identified in Upper Jurassic
deposits, from 150 million years ago (mya). However, while it does appear
intermediate between early reptiles and birds, there is some disagreement over
whether it is a direct ancestor of pre-sent day birds. Fossils unequivocally of
birds do not appear until the Cretaceous period, 80—120 mya, although the
number of species suggests that they radiated earlier. The earliest remains are
of large flightless diving birds, Hesperornis spp., with primitive teeth. Other
toothed sea birds also lived during the Cretaceous, including the flighted
ichthyosaurs. Also appearing in the Early Cretaceous were the Enantiornithes, a
little understood group of seemingly primitive birds. At the end of the period,
the toothed birds disappeared with the dinosaurs. Since then, only toothless
birds have been found in the record and it is not clear how or when they arose,
though it is thought that it was during the Cretaceous. By the Eocene (c. 50
mya), many modern forms were recognizable. These are non-passerines, including
ostriches, penguins, storks, ducks, hawks, cuckoos, and kingfishers. The
passerines (small songbirds) appear to have diversified 36—45 mya, along
withflowering plants and insects. Several other forms, mostly large birds, were
also present in the Eocene but died out. Other giant birds such as the larger
The evolutionary success of birds is evidenced by the wide variety of present-day forms. They have long been popular subjects of study for taxonomists. Traditional classifications are based mainly on morphological and anatomical differences in structure, plumage, and so forth. More recently, behavioral traits, song, and biochemical techniques (including DNA) have been employed. Yet, while there is general agreement as to the families to which the 9,000 or so extant bird species belong, a variety of opinions exists on the relationships within and between families.
Structure and function
adapted to a multitude of situations. For this reason, they occur in a wide
diversity of shapes, sizes, and col-ors. Weighing up to 285 lbs (130 kg), and
reaching 9 ft (2.75 m), the flightless ostrich is the largest of the living
birds. At a mere 0.7 oz (2 g) and around 2.4 in (6 cm), the tiny bee hummingbird
is the smallest. Even closely related forms can look very different (adaptive
radiation). A famous example is the enormous range of bill shapes and sizes in
Charles Dar-win's Galapagos finches; a single over-water colonizing species is
thought to have undergone repeated evolutionary divergence to produce the 14 or
so contemporary species on the different islands. Conversely, unrelated species
can closely resemble each other (convergent evolution) because they have evolved
for the same lifestyle. Examples of this are the
Body shapes vary enormously, from the flexible, long-necked form of the cranes and ibises to short-necked, stiff-backed falcons and penguins. These latter species, the speedy, predatory hunters of the air and the seas, have torpedo-shaped bodies to minimize drag. Bills and beaks take a variety of forms that generally reflect their major function in feeding: from the sturdy, seed-cracking bills of finches to the long, soil-probing between species, variation can be quite marked within species, either geographically or between the sexes.
Species often vary in size clinally (with environmental or geographic change), usually increasing in size between from hotter to cooler parts of their range (Bergman's rule); races at either end of the cline can be remarkably different. A few species even have different forms, for example, the large- and small-beaked snail kites. Males are often larger (sexual dimorphism), but in some species, including birds of prey, some seabirds, and game birds, the female is larger.
In a few species, bills also serve as signals of breeding condition and sexual ornaments to attract the opposite sex. For example, the bills of cattle egrets turn from yellow to orange-yellow in the breeding season and the huge gorgeously rainbow-hued bills of the sulphur-breasted toucans may separate species. Similarly, birds' feet and legs suit their lifestyle: webbed for swimming; short and flat for ground dwellers; longer and grasping for perching species; powerful and heavily taloned for raptorial species. Stilt-like legs and spider-like toes with a span the length of the bird's body are a feature of the lily-pad walking jacanas. The legs are almost nonexistent in swifts and other birds that spend much of their lives on the wing, and long and muscular in ostriches and emus that stride and run across the plains. The ostrich has two toes, and a few species such as those that run on hard surfaces have lost the first (hind) toe or it is very small. Most species have four toes but their arrangement differs: in most perching birds, toes two, three, and four point forward and the hind toe opposes them; some species have two toes pointing forward, two back; others can move a toe to have either arrangement; swifts have all four toes pointing forward.
Wings are less variable than lower limbs, although their different forms can be extreme: much reduced in the flight-less ratites, put to good use as fins in penguins, and at their most extended in gliding species that spend much of their lives riding air currents.
Birds' active lifestyles require highly developed senses. For the vast majority of species, sight is the dominant sense and the eyes are relatively large. The eyes are generally set to the sides of the head, allowing a wide field of view, (about 3000), presumably useful for detecting approaching predators. For predatory birds (insectivores and raptors), the eyes are set more forward to give a greater overlap in the field of vision of the two eyes. This increase in binocular vision is important for depth perception. Compared with mammals' eyes, birds' eyes are relatively immobile. They compensate by being able to rotate the head by as much as 270° in species such as owls, which have the most forward facing eyes. Their eyes are protected by a nictitating membrane, which closes from the inside to the outside corner, and a top and bottom eye-lid. Birds can focus their eyes rapidly, which is important in flight and when diving underwater. In general, they may not have exceptional visual acuity compared with humans. However, birds have a larger field of sharp vision, good color perception, and can also discriminate in the ultraviolet part of spectrum and in polarized light. Nocturnal species have more rods than cones in their retinas to enhance their vision in dim light.
The ear of birds is simpler than that of mammals, but their sense of hearing appears to be at least as sensitive. Some species, such as some of the owls, have a disc of stiff feathers around the face that directs sound to the ears, and asymmetrically placed ear openings and enlarged inner ears to enhance discrimination of direction and distance of the source of the sound. Oilbirds and some swiftlets that live in caves use echolocation. They emit audible clicks to help them navigate and locate prey in the dark.
number of sensory receptors and nerve endings distributed about the body
indicate that birds' sense of touch, pain, and temperature is keen. By contrast,
the olfactory system is poorly developed and few birds seem to make great use
of smell. Exceptions include the
Feathers distinguish birds from all other living animals (there is recent evidence that some dinosaurs were feathered but this remains controversial). Light, strong, and colorful, feathers are extraordinarily multifunctional. They provide warmth, protection from the elements, decoration and camouflage, and are specialized for aerodynamics and flight (most birds), hydrodynamics and diving (e.g., penguins), or to cope with both elements (e.g., cormorants). A few species use them to make sound (e.g., snipe) or carry water to their young (e.g., sandgrouse). Not least, they identify species and subspecies, may vary with age, sex and breeding condition, and signal emotion.
Feathers are made of keratin and, once grown, are entirely dead tissue. They are of six main types. The most obvious are the long, stiff feathers of the wings and tail that provide the flight surfaces; more flexible, contour feathers make thesculpted outer covering for the body; and down makes a soft insulative underlayer. Semi-plumes, which are between down and contour feathers in form, help to provide insulation and fill out body contours so that air (or water) flows easily over the body. Two types of feathers are mainly sensory in function: stiff bristles that are usually found around the face (around the feet in the Tyto owls) like the whiskers of a cat or a net around the gape of some insect-eating species, and filoplumes, which are fine, hair-like feathers with a tuft of barbs at the tip that lie beside contour feathers and monitor whether the plumage is in place. In some species, modified feathers form features such as crests, ornamental bristles, cheek tufts, plumes, and tail flags and trains.
The contour, flight feathers, and semi-plumes have a central shaft, and a vane made up of barbs and barbules that interlock with each other and sometimes with neighboring feathers. The bird carefully maintains these links by nibbling and pulling the feathers through its bill. Many birds bathe regularly, most in water, but a few in dust. Some such as herons and elanine kites have powder down that grows continuously and crumbles into a fine powder that is spread through the feathers for cleaning and water resistance. Other birds have oil glands at the base of the tail for the same purpose. Sunbathing also helps to maintain the health and curvature of feathers. Some birds, including many passerines, appear to use biting ants in feather maintenance, perhaps to control ectoparasites, either by wallowing among the swarm or by wiping individual ants through their plumage.
Over time, feathers become worn and bleached, and dam-aged by parasites. They are completely replaced annually in most except very large birds such as eagles and albatrosses, which spread the molt over two or so years. Some species, notably those that change from dull winter plumage into bright breeding colors (e.g., American goldfinch), have two molts a year: a full molt after breeding, and a partial body molt into breeding plumage. Most species shed their feathers sequentially to maintain their powers of flight. However, a few, particularly waterbirds that can find food in the relative safety of open water, replace all their flight feathers at once and are grounded for about five weeks.
Spectacular colors are a feature of birds, from the soft, mottled leaf patterns of nightjars (cryptic) to the gaudy, ornate plumes of a peacock (conspicuous). Cryptic colors conceal the bird from predators or rivals; conspicuous colors are used in courtship or threat. The colors themselves are produced by pigments in the feathers themselves or by structural features that interact with the pigment and the light to produce iridescent color, which can only be seen from certain angles, or non-iridescent color, that can be seen from any angle. In many species, the sexes are similar in color. In others, the sexes differ, and usually the male is showier and the female resembles a juvenile bird. In these species, sexual selection is thought to have favored dichromatism (and dimorphism) through female preference for partners with bright colors (and extravagant ornamentation). In the few polyandrous species, the reverse is that case and the females are more vibrant. Plumage may also vary geographically, with races from warm, humid climates tending to be more heavily pigmented than those from cool, dry regions (Gloger's rule).
Anatomy and physiology
The skeleto-muscular system of birds combines light weight with high power for flight. Muscle mass is concentrated near the center of gravity—around the breast and basesof the wings and legs—which gives a compact, aerodynamic form. Long tendons control movements at the ends of the limbs. Flighted birds have more massive breasts and wing muscles; in terrestrial birds, much of the muscle mass is in the upper legs. In perching birds, the tendon from the flexor muscle loops behind the ankle; when the ankle bends on landing, the toes automatically close around the perch and maintain the grip without effort, anchoring the bird even in sleep. In many species, the toe tendons have ridges, which also help to lock the feet around the perch.
In contrast to the mammalian skeleton, birds' bones are hollow and less massive and several have fused to form a strong, light frame. A bird's skeleton constitutes only about 5% of its mass. Another distinctive feature is that the bones, including the skull, are pneumatized: their core is filled with air via a system of interconnecting passages that connect with the air sacs of the respiratory system and nasal/tympanic cavities. Flighted species tend to have extensive pneumatization but it is reduced or lacking in diving birds, which would be hindered by such buoyancy. Even birds' bills are light—the horny equivalent of the heavy, toothed muzzle of mammals.
Respiration, circulation, and body temperature.
Unlike mammals, birds lack a diaphragm. Instead, air is drawn into the rigid lungs by bellow-like expansion and con-traction of the air sacs surrounding the lungs and another group in the head, which is driven by muscles that move the ribs and sternum up and out and back again. Features of birds' circulatory and respiratory systems make their respiration more efficient than that of most mammals, allowing them to use 25% more oxygen from each breath. This enables them to sustain a high metabolic rate and, among other benefits, assists those high-flying migrants that cross the world's tallest mountains and reach altitudes up to 29,500 ft (9,000 m), where the oxygen content of the air is low.
Birds and mammals both generate their own body heat, but birds' high metabolic rate helps to maintain theirs at around 100°F (38–42°C), depending on species; 5–7°F (3–4°C) hotter than most mammals. When it becomes difficult to obtain enough energy to stay warm and active, a few species become torpid (lower their body temperature and become inactive) on the coldest days or during bad weather, usually for a few daysor overnight. Other birds cope with cold by increasing their metabolic rate slightly, growing denser feathers, having a layer of fat, or by behavioral means such as huddling with others, tucking up a leg to decrease the heat loss surface, and fluffing out the feathers to trap more air. Lacking sweat glands to shed body heat, they may pant, lower their metabolic rate, seek shade, or raise their feathers to catch the breeze in hot weather.
Digestion and excretion
To provide the energy needed for flight, and so that they are not weighed down for a long time by the food they have consumed, birds have a high metabolic rate and digest food rapidly and efficiently. Their digestive tract is modified accordingly, and many species have the second part of their two-part stomach modified into a muscular gizzard where hard food is physically ground down so that gastric juices can penetrate easily. Some species swallow pebbles to assist with this break-down. The digestive tract tends to be long in grazers, fish-eaters, and seed-eaters, and short in meat- and insect-eaters.
Birds have three ways to rid themselves of excess water, salts, and waste products: through breathing and the skin; the renal system; and salt glands. The salt glands are located in the orbit of each eye. They secrete sodium chloride and, therefore, are well developed in seabirds that have a salty diet, and non-functional in some other groups. Birds' kidneys are more complex than those of mammals: they produce concentrated urine with nitrogen waste in the form of insoluble uric acid, rather than urea. Such a water-efficient system does not require a (heavy) bladder, with obvious advantages for flight. It also enables many species, especially those with a moist diet (carnivores, insectivores, and frugivores), to drink seldom or not at all. Both the white urine and dark fecal mat-ter are voided through the anus, and bird droppings often contain both.
Life history and reproduction
To a large extent, body size determines life history. Compared with smaller species, larger species tend to live longer, breed at a later age, have a longer breeding cycle, and, at each breeding attempt, produce fewer young with a greater chance of survival. There are always exceptions, and climate and risk of predation and other factors impinge on this overlying pat-tern. For example, some small temperate zone Australian passerines live up to 18 years and have small clutches, whereas ground-nesting grouse may live a few years and have large clutches. For convenience, species may be classed as fast-breeders (r-selected), which have many large clutches and short periods of nestling care, and slow-breeders (K-selected) that have a few small clutches and extended periods of offspring care. In reality, there is a continuum between the two extremes.
majority of bird species are at least socially monogamous, that is, a pair of
birds cooperates to raise young. They may stay together for the breeding attempt
or mate for life. However, many other arrangements exist. Some birds have a
polygynous mating system, particularly species that use rich resources that are
clumped, so that a male can support more than one female (e.g.,
Most birds nest as solitary pairs, but some 13% of species are colonial, particularly seabirds. Colonies may be a few pairs (e.g., king penguin) or millions (e.g., queleas). In between are species that nest in loose colonies, either regularly or when conditions are favorable. Nest spacing varies enormously from a few nches/centimeters in some colonies to several miles/kilometers in species that have large territories. Spacing is linked to food and nest sites; nests are closer together where resources are plentiful.
Breeding seasons and nests
Most birds have a regular breeding season, timed to coincide with the most abundant season of the year, but some, particularly those adapted to unpredictable climates, are opportunistic, only breeding when conditions allow. The largest species may breed every two years, but most attempt to breed at least annually, some raising several broods over the breeding season.
Birds build their nests of several substrates; the most important issues are protection from predators and the elements. Therefore, species that nest on predator-free islands often build close to the ground but, on the mainland, species build in higher locations. The nest itself must hold and shelter the eggs; in form, they vary enormously from simple scrapes in the dirt to large complex stick nests and hanging structures, woven together or glued with cobwebs or mud, and lined with soft material. Tree holes and holes in banks or cliffs also make good nests but competition for them may be fierce. The megapodes construct a mound in which they bury their eggs and maintain the temperature at about 90—95°F (32—35°C) by scratching soil on or off. Many species use the same nest area, nest site, or actual nest year after year.
Eggs and incubation
Birds' eggs are beautiful in their variety. They may be plain or colored, marked or unmarked, oval or round. All are slightly more pointed at one end so that the egg tends to roll in a circle. Species that lay in the open where eggs may roll tend to have long-oval, cryptically colored eggs, and those that lay in holes tend to have rounder, unmarked eggs. Egg laying can be energetically costly, especially for small birds: for a hummingbird, each egg (0.01 oz/0.3g) represents 25% of the bird's mass; for an ostrich, the 50 oz (1,500 g) egg is 1% of the hen's weight. Birds that have precocial chicks tend to have large eggs (about 35% yolk compared with 20% in altricial [more help-less] species) because the chick must be advanced and well-developed when it hatches. Clutch size varies enormously both within and between species. Nevertheless, most species have a typical number of eggs; one in the kiwi, perhaps 20 in some ducks. Larger species tend to have fewer eggs. In some species, two are laid but only one ever hatches. Across species, there tends to be a trade-off between egg-size and clutch-size, some species lay a few large eggs, others many smaller eggs. In some species, the clutch size is fixed (determinate layers), in others, if the eggs are lost or removed within the breeding season, the bird will go on laying eggs (indeterminate layers). The majority of species lay every second day until the clutch is complete; in a few of the largest species, the interval is four days. In the vast majority of species incubation is carried out by one or both of the parents, but a few species, such as some ducks, nest-dump (lay some of their eggs in a neighbor's nest), and some such as the cuckoos are parasites and lay their eggs in the nest of an-other species. Incubation varies little within species. Among species, it ranges in length from 10 days for some woodpeckers to about 80 days for albatross and the very large-egged kiwi.
Egg formation and embryo development
To most people, eggs and birds go together. Certainly all bird species lay shelled eggs, but so too do some reptiles. The egg contains nutrients for the developing embryo. There is some exchange of gases and water across the shell, and waste from the embryo is stored in a sac that develops outside of the embryo, but within the shell (the allantois). An-other sac develops into an air sac for ventilation. Once incubation begins, development is rapid, triggered by heat either from a brooding parent, as in most birds, or the environment, as in mound builders, which bury their eggs to capture heat from the sun. Within days, the embryo has large eyes and rudimentary organs. As it develops, the yolk sac is absorbed and the embryo fills more of the shell. By hatching, the yolk is fully absorbed and the embryo has moved its bill into the air sac and begins to breathe air. By this time, mainly through water loss, the egg is about 12–15% lighter than at the start of incubation. The embryo uses an egg tooth on the tip of its bill to chip away a ring around one end of the shell (already thinned by loss of calcium to the embryo) and hatch. If it is altricial (e.g., songbirds and seabirds), the chick will be naked or downy, and helpless; the chick of a precocial species (e.g., ostriches, ducks, and game birds) will be feathered, able to regulate its own body temperature, and can follow its parent and feed it-self almost immediately. The chicks of precocial species hatch synchronously (at the same time), those of other species are sometimes asynchronous, resulting in a mixed-age brood.
Growth and care of young
Most birds grow remarkably fast and, in many altricial species, are more or less fully grown by the time they leave the nest. At the extremes, chicks stay in the nest about 10–20 days most bird species, only the left ovary develops. The ovary holds a large number of oocytes. During the breeding season, a few oocytes (immature eggs) start to develop. They are covered in a follicle that lays down yolk, which is manufactured in the liver and carried in the blood. The one with the most yolk is shed first; the follicle ruptures and the oocyte moves into the first part of the oviduct where it may be fertilized from sperm stored in the sperm storage glands. During about 20 hours it passes down the oviduct where it is covered in albumin and, towards the end, the outer layer calcifies to form the shell and any markings are laid down from blood (red-brown) or bile (blue-green) pigments. About this time, if an-other egg is to be laid, another oocyte is released. In a few large species, the process may take longer in passerines and 150–250 days in albatrosses. Precocial species, which are free of the nest, are slower growing—their rate of growth is approximately one-third that of altricial species.
The amount and type of care given is diverse. Nidicolous species, in which the chicks stay in the nest, put in considerable effort bringing food to the nest, either bringing many small items (e.g., insectivores) or a few large ones (raptors). Some species regurgitate food for the chicks (e.g., seabirds). These species must also keep the chicks warm and dry and protect them from predators, and may also keep the nest free of droppings. Nudifugous species, in which the chicks follow adult(s), are either fed by the adult (some seabirds) or, more commonly, feed them-selves on plant material (e.g., many waterfowl and game birds).
Parental care varies accordingly. As protection against predators, some waterfowl carry their young on their backs and, in colonial species, several neighbors unite to try to drive off an intruder. In some species, the breeding pair is accompanied by "helpers," which may be offspring from earlier breeding at-tempts or unrelated individuals, males or females or both sexes, juveniles or adults. Helpers help particularly in those species in which prey is difficult to find or catch.
All species have a niche—the range of environmental conditions under which they can survive and reproduce. The few birds have the ability to digest cellulose. Not surprisingly, birds are pollinators and seed dispersers for a great variety of plants. Some species specialize, others are more varied in their diet. Methods of collecting food are numerous, but most involve sight.
Birds are infected by a variety of diseases, both internal and external, caused by bacteria, viruses, and parasites. Some of these can also affect humans and their livestock. In general, healthy birds can carry both internal and external parasites without obvious harm. Nevertheless, disease outbreaks occur; for example, following floods, outbreaks of insect-borne pox can occur in wild birds. Birds can also suffer from exposure to toxic substances of both natural (e.g., toxic algal blooms) and human-made origin (e.g., oil spills, pesticides, lead-shot, and other pollutants). Predation is a fact of life for the majority of bird species, particularly for their eggs and young. Their life history has evolved to allow for losses, which are naturally high. For example, in passerines, perhaps 5% of eggs result in adult birds and annual mortality of adults may be as high as 60%. For larger species, the rate of mortality tends to be lower. Provided that they are not too prolonged or severe, starvation, predation, disease, and other causes of niche may be broad and unspecialized or narrow and highly specialized. Coexisting species tend not to overlap much in their niche requirements. This is particularly obvious with food: the type and where, when, and how it is collected. For example, three hawks coexist in some Australian woodlands and they roughly segregate by species and sex: the small male sparrow hawk takes passerines in the canopy; the medium-sized brown goshawk hunts birds and small mammals in the air and on the ground in more open spaces; and the largest, the female gray goshawk, captures medium-sized birds and mammals on or near the ground.
energy for flight, birds need highly nutritive food. For this reason, most birds
eat at least some arthropods, especially insects. They may do this
incidentally, for example, mixed in with nectar, but mostly they are actively
captured. In addition to this, there are four basic diet groups: the carnivores
(those that eat other vertebrates, including the fish-eaters); berry- and
seedeaters; eaters of non-flowering plants (fungi, mosses, algae, and so forth);
eaters of flowering plants (roots, tubers, leaves, nectar). Grasses and herbage
are low in nutritive value and must be consumed in too-large quantities to be of
major importance to many species, and 31 endemic families. The Nearctic (
Bird populations vary enormously in their abundance and density. Some species are widely scattered across the landscape (e.g., the solitary eagles), others live in crowded colonies of millions (queleas). There are several general patterns: population density is related to body size (smaller species tend to be more numerous); the number of species and of individuals tends to be greatest in complex habitats (e.g., forest compared with grassland) and where several habitats meet (ecotones); the number of species increases with the size of the habitat patch; and, the number of species and overall densities tend to increase from the poles to the equator.
Bird species can be social or solitary; they may nest, roost, and feed in small or large groups for part of their lives (e.g., when young or in the nonbreeding season) or their entire lives. Some form feeding flocks with other bird species or nest near more powerful species for protection. Some associate loosely with other vertebrates, such as following monkey troops to catch the insects they flush. A few live more or less mortality are compensatory and, in general, bird populations recover quickly.
Distribution and biogeography
distributed across all continents and on most is-lands, and in all major
habitats from caves to mountaintops, deserts to rainforests. Many factors limit
the distribution of species, all relating to their ecology: climate, habitat
avail-ability, and the presence of predators, competitors, and food. There may
also be physical barriers such as mountain ranges, oceans, and impassable
expanses of unsuitable habitat. The breeding range often differs from the
nonbreeding range be-cause of seasonal or other movements to remain in the most
favorable conditions. There are broad patterns to general distribution; for
example, woodpeckers are found in many regions of the world but are absent from
Birds have developed complex communication systems, including various calls and song and visual signals involving posture, movements, facial expressions, display of certain characteristics of plumage, and, in some species, flushing of the skin (e.g., vultures) or popping of the eyes (Australian choughs). Some behaviors, like the dance of brolgas and synchronized swimming of swans, is ritualized, others appear more spontaneous. Courtship is a highly ritualized sequence of displays, on the ground or in the air, and can involve court-ship feeding, which may be a way for females to judge the quality of their partner.
As a group, birds are cerebrally advanced. Much of their behavior is innate but they also learn by experience through-out life. They have good memories, such as retrieving cached food items from several stores several weeks after they were hidden. Some species appear to be particularly adaptable if not intelligent. Certainly, their behavior can be complex and interesting. Individual rooks wait to cache nuts if a non-hoarding rook is passing by, yet are unconcerned by rooks that are also hoarding. Jays that steal food are more likely to move their caches to prevent theft than are jays that are not thieves. Parrots have a complex social system and enjoy playing with each other and with found objects. Black kites and green-backed herons have learned to place bread on water to attract fish. One of the Galapagos finches uses a cactus thorn to probe bark crevices for insects that it cannot reach with its short tongue.
edited by Melissa C. McDade (Gale Group) Excerpt: At first sight, this is not a
difficult question. Every child is able to identify an animal as a mammal. Since
its earliest age it can identify what is a cat, dog, rabbit, bear, fox, wolf,
monkey, deer, mouse, or pig and soon experiences that with any-one who lacks
such a knowledge there would be little chance to communicate about other things
as well. To identify an animal as a mammal is indeed easy. But by which
characteristics? The child would perhaps explain: Mammals are hairy four-legged
animals with faces.
Against expectation, the three characteristics reported by this naive description express almost everything that is most essential about mammals.
Hair, or fur, probably the most obvious mammalian feature, is a structure unique to that group, and unlike the feathers of birds is not related to the dermal scales of reptiles. A mammal has several types of hairs that comprise the pelage. Specialized hairs, called vibrissae, mostly concentrated in the facial region of the head, perform a tactile function. Pelage is seasonally replaced in most mammals, usually once or twice a year by the process called molting. In some mammals, such as ermines, the brown summer camouflage can be changed to a white coat in winter. In others, such as humans, elephants, rhinoceroses, naked mole rats, and aardvarks, and in particular the aquatic mammals such as walruses, hippopotami, sirenia, or cetaceans, the hair coat is secondarily reduced (though only in the latter group is it absent completely, including vibrissae). In the aquatic mammals (but not only in them), the role of the pelage is performed by a thick layer of subcutaneous adipose tissue by which the surface of body is almost completely isolated from its warm core and the effect of a cold ambient environment is substantially reduced. Thanks to this tissue, some mammals can forage even in cold arctic waters and, as a seal does, rest on ice without risk of freezing to it. In short, the essential role of the sub-cutaneous adipose layer and pelage is in thermal isolation, in preventing loss of body heat. Mammals, like birds, are endotherms (heat is generated from inside of the body by continuous metabolic processes) and homeotherms (the body temperature is maintained within a narrow constant range).
The body temperature of mammals, about 98.6°F (37°C), is optimal for most enzymatic reactions. A broad variety of functions are, therefore, kept ready for an immediate triggering or ad hoc mutual coupling. All this also increases the versatility of various complex functions such as locomotion, defensive reactions, and sensory performances or neural processing of sensory information and its association analysis. The constant body temperature permits, among other things, a high level of activity at night and year-round colonization of the low temperature regions and habitats that are not accessible to the ectothermic vertebrates. In short, endothermy has a number of both advantages and problems. Endothermy is very expensive and the high metabolic rate of mammals re-quires quite a large energetic intake. In response, mammals developed a large number of very effective feeding adaptations and foraging strategies, enabling them to exploit an extreme variety of food resources from insects and small vertebrates (a basic diet for many groups) to green plants (a widely accessible but indigestible substance for most non-mammals). At the same time, mammals have also developed diverse ways to efficiently control energy expenditure.
Besides structural adaptations such as hair, mammals have also developed diverse physiological and behavioral means to prevent heat and water loss, such as burrowing into underground dens; seasonal migrations or heterothermy; and the controlled drop of body temperature and metabolic expenditure during part of the day, or even the year (hibernation in temperate bats, bears, and rodents as well as summer estivation in some desert mammals). So, considerable adaptive effort in both directions increases foraging efficiency and energy expenditure control. When integrated with morphological, physiological, behavioral, and social aspects, it is an essential feature of mammalian evolution and has contributed to the appearance of the mammalian character in many respects.
Four legs, each with five toes, are common not only to many mammals, but to all terrestrial vertebrates (amphibians, rep-tiles, birds, and mammals), a Glade called Tetrapoda. Nevertheless, in the arrangement of limbs and the modes of locomotion that it promotes, mammals differ extensively from the remaining groups. The difference is so clear that it allows us to identify a moving animal in a distance as a mammal even in one blink of an eye. In contrast to the "splayed" reptilian stance (i.e. horizontal from the body and parallel to the ground), the limbs of mammals are held directly beneath the body and move in a plane parallel to the long axis of the body. In contrast to reptiles, whose locomotion is mostly restricted to the lateral undulation of the trunk, mammals flex their vertebrate column vertically during locomotion. This arrangement enables a powered directional movement, such as sustained running or galloping, very effective for escaping from a predator, chasing mobile prey, or exploring spatially dispersed food resources. The respective rearrangements also bring another effect. By strengthening the vertebral column against lateral movement, the thoracic cavity can be considerably enlarged and the thoracic muscles released from a locomotory engagement, promoting changes to the effective volume of the thoracic cavity. With a synergetic support from another strictly mammalian structure, a muscular diaphragm separating the thoracic and visceral cavity, the volume of the thoracic cavity can change during a breathing cycle much more than with any other vertebrates. With the alveolar lungs, typical for mammals, that are designed to respond to volume changes, breathing performance enormously increases. This enables a mammal to not only keep its basal metabolic rate at a very high level (a prerequisite for endothermy) but, in particular, to increase it considerably during locomotion. In this connection, it should be stressed that the biomechanics of mammalian locomotion not only allow a perfect synchronization of limb movements and breathing cycles but, with the vertical flex of the vertebral column, are synergetic to the breathing movements and support it directly. As a result, the instantly high locomotory activity that characterizes a mammal increases metabolic requirements but at the same time helps to respond to them.
The face is the essential source of intra-group social information not only for humans but for many other mammal groups. The presence of sophisticated mechanisms of social integration and an enlarged role in interindividual discrimination and social signaling are broadly characteristic of mammals. Nevertheless, each isolated component contributing to the complex image of the mammalian face says something important regarding the nature of the mammalian constitution, and, moreover, they are actually unique characters of the group. This is particularly valid for fleshy cheeks and lips, the muscular belt surrounding the opening of a mouth. The lips and the spacious pocket behind them between the cheeks and teeth (the vestibulum oris) are closely related to feeding, and not only in that they enlarge the versatility of food processing in an adult mammal. The lips, cheeks and vestibulum oris are completely developed at the time of birth and since that time have engaged in the first behavioral skill performed by a mammal. Synergetic contraction of lip and cheek muscles producing a low pressure in the vestibulum oris is the key component of the suckling reflex, the elementary feeding adaptation of a newborn mammal. All mammals, without exception, nourish their young with milk and all female mammals have large paired apocrine glands specialized for this role—the mammary glands, or mammae. Nevertheless, not all mammalian newborns actually suck the milk. In the egg-lying monotremes (the Australian duck-billed platypus and spiny anteaters), mammary glands lack the common milk ducts and nipples, so young do not suck but instead lick the milk using their tongue. All other mammals, both marsupials and eutherians, together denoted as Theria, bear a distinctive structure supporting suckling—the paired mammary nipples. The nipples originate independently from mammary glands, they are present both in males and females, and their number and position is an important character of individual clades. The therian mammals are all viviparous. For the most vulnerable period of their lives they are protected first by the intrauterine development with placental attachment of the embryo and then by prolonged postnatal parental care. A milk diet during the latter stage postpones the strict functional control on jaws and dentition and enables postnatal growth, the essential factor for the feeding efficiency of an adult mammal. At the same time this provides extra time for development of other advanced and often greatly specialized mammalian characteristics: an evolving brain and the refinement of motor capacities and behavioral skills. Thanks to the extended parental investment that mammalian offspring have at the be-ginning of their independent life, they enjoy a much higher chance for post-weaning survival than the offspring of most other vertebrates. The enormous cost of the parental investment places, of course, a significant limit upon the number of offspring that can be produced. Despite the great variation in reproductive strategies among individual mammalian clades, in comparison to other vertebrates (excepting elasmobranchians and birds), the mammals are clearly the K-strategists (producing few; but well-cared for, offspring) in general.
The other components of the mammalian face provide correspondingly significant
information on the nature of these animals. The vivid eyes with movable eyelids,
external auricles, nose, and last but not least long whiskers (vibrissae, the
hairs specialized for tactile functions), show that a mammal is a sensory
animal. Most extant mammals are noctural or crepuscular and this was almost
certainly also the case with their ancestors. In contrast to other tetrapods,
which are mostly diurnal and perceive almost all spatial information from
vision, mammals were forced to build up a sensory image of the world from a
combination of different sources, in particular olfaction and hearing.
Nevertheless, vision is well developed in most mammals and is capable of very
fine structural and color discrimination, and some mammals are secondarily just
optical animals. For example, primates exhibit a greatly enlarged capability
for stereoscopic vision. In any case, all mammals have structurally complete
eyes, though the eyes may be covered by skin in some fossorial mammals (such as
blind mole rats, or marsupial moles) or their performance may be reduced in
some respect. In comparison with other vertebrates, the performance of vision is
particularly high under low light intensities, and the eyes are quite mobile.
The latter character may compensate for a reduced ability of head rotation in
mammals due to the bicondylous occipital joint contrasting to a monocondylous
joint in birds or reptiles. The eyes are covered by movable eyelids (not
appearing in reptiles), significant both in protecting the eyes and in social
signaling. The remaining two structures—nose and auricles—are particularly
unique for mammals and are related to the senses that are especially important
for mammals: olfaction and hearing. Not only the nose and auricles themselves,
but also the other structures associated with the senses of smell and hearing
feature many traits unique to mammals.
The other components of the mammalian face provide correspondingly significant information on the nature of these animals. The vivid eyes with movable eyelids, external auricles, nose, and last but not least long whiskers (vibrissae, the hairs specialized for tactile functions), show that a mammal is a sensory animal. Most extant mammals are noctural or crepuscular and this was almost certainly also the case with their ancestors. In contrast to other tetrapods, which are mostly diurnal and perceive almost all spatial information from vision, mammals were forced to build up a sensory image of the world from a combination of different sources, in particular olfaction and hearing. Nevertheless, vision is well developed in most mammals and is capable of very fine structural and color discrimination, and some mammals are secondarily just optical animals. For example, primates exhibit a greatly enlarged capability for stereoscopic vision. In any case, all mammals have structurally complete eyes, though the eyes may be covered by skin in some fossorial mammals (such as blind mole rats, or marsupial moles) or their performance may be reduced in some respect. In comparison with other vertebrates, the performance of vision is particularly high under low light intensities, and the eyes are quite mobile. The latter character may compensate for a reduced ability of head rotation in mammals due to the bicondylous occipital joint contrasting to a monocondylous joint in birds or reptiles. The eyes are covered by movable eyelids (not appearing in reptiles), significant both in protecting the eyes and in social signaling. The remaining two structures—nose and auricles—are particularly unique for mammals and are related to the senses that are especially important for mammals: olfaction and hearing. Not only the nose and auricles themselves, but also the other structures associated with the senses of smell and hearing feature many traits unique to mammals.
Mammals construct much of their spatial information with the sole aid of olfactory, acoustic, or tactile stimuli combined with information from low-intensity vision. This task necessitated not only a considerable increase in the capacity and sensory versatility of the respective organs, but also the refinement of the semantic analysis of the information they provide. As a result, the brain structures responsible for these tasks are greatly enlarged in mammals. The tectum mesencephali, a center for semantic analysis of optical information bi-lobed in other vertebrates, is supplemented by a distinc center of acoustic analysis by which the tectum of mammal: becomes a four-lobed structure, the corpora quadrigemina The forebrain or telencephalon, a structure related to olfac. tory analysis, is by far the largest part of the mammalian brain Its enlargement is particularly due to the enlarging of the net:, cortex, a multi-layered surface structure of the brain, which further channels inputs from other brain structures and play the role of a superposed integrative center for all sensory sensory-motor, and social information.
Many of the characters common to mammals do not appear in other animals. Some of them, of course, can be observed also in birds—a very high (in respect to both maximum and mean values) metabolic rate and activity level or complexity of particular adaptations such as advanced parental care and social life, increased sensory capacities, and new pathways of processing sensory information or enormous ecological versatility. Fine differences between birds and mammals suggest that the respective adaptations are homoplasies—that is, they evolved in both groups independently.
Other mammalian characteristics are synapomorphies of Amniota, the characteristics shared because of common ancestry. The amniotes, a group including reptiles, birds, and mammals, are the terrestrial vertebrates in which embryonic development takes place under the protection of fetal membranes (amnion, chorion, allantois). As in other amniotes, mammals are further characterized by an increased role of parental investment, internal fertilization, keratinized skin derivatives, an advanced type of kidney (metanephros) with a specific ureter, an advanced type of lung respiration, and the decisive role of dermal bones in skull morphology. Of course, at the same time, mammals share a large number of characteristics with all other vertebrates, including the general body plan, solid inner skeleton, the design of homeostatic mechanisms (including pathways of neural and humoral regulation), and functional integration of particular developmental modules. Mammals also share with other vertebrates the patterns of segmentation of trunk skeleton and muscles and the specific arrangements of the homeobox genes organizing the body segmentation as well as a lack of their expression in the head region, etc. These characters are synapomorphies of vertebrates, which are at least partly retained not only in some amniotes but throughout all other vertebrate clades. With respect to mammals, these are symplesiomorphies, the primitive characters that do not reveal closer relations of the class but on its broadest phylogenetic context.
Mammals also exhibit a large number of qualities that are fully unique to them, the autapomorphies. The autapomorphies are the characteristics by which a taxon can be clearly distinguished and diagnosed. Thus, though many characteristics of mammals are not specific just to them, answering the question "what is a mammal?" means first demonstrating the autapomorphies of that group. A simplified list of them includes:
(1) The young are nourished with milk produced by (2) mammary glands. These glands appear in all female mammals, and are the structure from which the class Mammalia got its name. (3) Obligatory vivipary (in Theria, i.e., marsupials and placentals) is the reproductive mode with a specialized organ inter-connecting the embryo and maternal tissues, the chorioallantoic placenta (in Eutheria, i.e., placentals). (4) Hairs, covering the body, grow from deep invaginations of the germinal layer of epidermis called follicles. Similar to other amniotes, the hair is composed of keratin and pigments, but its structure is unique for mammals. (5) Skin is rich in various glands. Most mammals have sweat glands (contributing to water balance and cooling the body surface), scent glands, and sebaceous glands. (6) The specific integumental derivatives, characteristic of particular groups of mammals, are composed either exclusively of keratin (such as claws, nails, and hoofs, which protect the terminal phalanx of the digits and adapt them to a specific way of locomotion or foraging) or of keratin in combination with dermal bone structures (horns of bovids and antlers of cervid artiodactyls, which play a considerable role in social signaling). A large variety of integumental derivatives are included in defensive adaptations: dermal armors of armadillos or keratinized scales of pangolins, spines modified from hairs in echidnas, hedgehogs, tenrecs, porcupines, or spiny mice, or the accumulations of hairlike fibers keratinized into a horn structure in rhinoceroses. (7) Limb position and function are modified to support specific locomotory modes of mammals such as jumping, galloping, or sustained running and can be specifically rearranged. The extreme rearrangements are seen in bats, which fly using a forelimb wing, and in specialized marine mammals, pinnipedian carnivores, cetaceans, and sirenia, whose forelimbs take the shape of a fin (the external hind limbs are absent in the latter two groups). (8) Pectoral girdle is simplified in comparison to the non-mammalian state: coracoid, precoracoid and interclavicle bones are lost (except for monotremes, which retain them) or partly included in the scapula. Also the clavicle, the last skeletal element that fixes the limb to the axial and thoracic skeleton, is lost in many groups. With these rearrangements the forelimbs get new locomotory qualities (such as extensive protraction), supporting abilities such as climbing and fine limb movements and providing a new spectrum of manipulative functions from cleaning hair to a variety of prey manipulations. (9) The bones of the pelvic girdle are fused into a single bone, with enlarged and horizontally prolonged ilium.
(10) A great degree of regional differentiation of the vertebral column. All mammals (except some edentates and manatees) have seven cervical vertebrae with the first two (atlas and axis) specifically rearranged to support powered head movements. (11) The vertebral column is strengthened against lateral movements but is greatly disposed to the vertical flexion. This is seen first of all in the lumbar section, whose vertebrae, in contrast to the non-mammalian ancestors, lack ribs. (12) The mammalian skull is bicondylous (the first vertebra, atlas, joints the skull via paired occipital condyles located on the lateral sides of the large occipital foramen), with (13) an enlarged braincase, (14) massive zygomatic arches (formed by the jugale and squamosum bones), and (15) a spacious nasal cavity with a labyrith of nasal turbinalia covered by vascularized tissue important both for olfaction (ethmoidal turbinalia) and/or heat and water exchange during breathing (maxillary turbinalia). (16) The nostrils open at a common structure called the nose, obviously the most prominent point of the head. The ancestral form of the nose, the rhinarium, is a hairless field of densely circular-patterned skin surrounding the nostril openings. The rhinarium is particularly large in macrosmatic (highly developed sense of smell) mammals (such as carnivores or artiodactyls), in lagomorphs, some rodents, and bats. In strepsirhine primates it is incised by a central groove, the phlitrum, while in some other groups such as in macroscelids or in elephants, the nose is prolonged and attains a number of supplementary functions. In contrast, all these structures are absent in cetaceans in which the nasal cavity is reduced and the nostrils (or a single nostril opening in Odontoceti) appear at the top of the head and their function is restricted to respiration. (17) Left and right maxillary and palatal bones are fused in early development and form the secondary bony palate, which is further extended by a fleshy soft palate. These structures provide a complete separation of the respiratory and alimentary tracts. The early appearance of such a separation is one of the essential prerequisites for suckling milk by a newborn and, hence, it seems probable that the secondary palate first appeared simply as an adaptation for this. (18) The heart is a large four-chambered organ (as in birds) with the left aorta persistent (not the right one, as in birds). (19) Erythrocytes, the red blood cells, are biconcave and lack nuclei. Thrombocytes are transformed to nonnucleated blood platelets.
(20) Lungs have an alveolar structure, ventilated by volume changes performed by the counteraction of two independent muscular systems, and a (21) muscular diaphragm, unique for mammals. (22) The voice organ in the larynx, with several pairs of membranous muscles, is unique for mammals. It is capable of very specialized functions such as the production of various communicative signals or high-frequency echolocation calls in bats and cetaceans. (23) There are three ossicles in the middle ear (malleus, incus, stapes). The former two are unique to mammals and are derived from the elements of the primary mandibular joint—articulare and quadratum—which still retain their original function in the immediate mammalian ancestors. The third bone of the primary mandibular joint, the angulare, changes in mammals into the tympanic bone, which fixes the tympanic membrane and finally enlarges into a bony cover of the middle ear—the bulae tympani. (24) The sound receptor (Corti's organ of the inner ear) is quite long and spirally coiled in mammals (except for monotremes) and surrounded by petrosum, a very compact bone created by a fusion of several elements. (25) With an enlarged braincase, the middle ear and tympanic membrane are thus located deeper in the head and open to the external environment by a long auditory meatus terminating with (26) a large movable external auricle. Auricles (pinnae) are specifically shaped in particular clades and contribute to the lateral discrimination of the auditory stimuli and directionality of hearing. They may be absent in some aquatic mammals (cetaceans, sirenia, walruses), while they are extremely pronounced and diversified in other groups such as bats, for which the acoustic stimuli (echoes of the ultrasonic calls they emit) are by far the most important source of spatial information. (27) In contrast to other amniotes, the lower jaw, or mandible, is composed of a single bone, dentary or dentale, which directly articulates with the temporal bone of the skull at the (28) dentary-squamosal joint. This arrangement not only fastens the jaw joint to resist the forces exerted during strong biting but also simplifies the functional rearrangements of jaw morphology responding to different demands of particular feeding specializations. (29) In all mammals, the posterior part of the mandible extends dorsally into the ramus mandibulae, which provides an area of attachment for the massive temporal muscles responsible for the powered adduction of the mandible.
(30) Essentially, all mammals have large teeth despite considerable variation in number, shape, and function in particular groups and/or the fact that some mammals secondarily lack any teeth at all (anteaters of different groups, and the platypus). Teeth are deep-rooted in bony sockets called alveoles. Only three bones host the teeth in mammals: the pre-maxilla and maxilla in the upper jaw and the dentary in the lower jaw. (31) Mammalian dentition is generally heterodont (of different size, shape, etc.). Besides the conical or unicuspidate teeth (incisors and a single pair of canines in each jaw) mammals also have large complex multicuspidate molars (three in placentals, four in marsupials, in each jaw quadrant) and pre-molars situated between canines and molars whose shape and number varies considerably among particular groups. The latter two teeth types are sometimes called "postcanines" or "cheek teeth." (32) The molars are unique to mammals. The basic molar type ancestral to all particular groups of mammals is called tribosphenic. It consists of three sharp cones connected with sharp blades. In combination with the deep compression chambers between blades, such an arrangement provides an excellent tool both for shearing soft tissues and crushing insect exoskeletons. This type of molar is retained in all groups feeding on insects, such as many marsupials, tenrecs, macroscelids, true insectivores such as moles, shrews or hedgehogs, bats, tree shrews, and prosimian primates, but the design of the molar teeth is often extensively rearranged in other groups. The multicuspidate structure of molars bears enormous potential for morphogenetic and functional re-arrangements, one of the prerequisites of the large diversity of feeding adaptations in mammals. (33) Mammalian dentition is diphyodont. This means that there are two generations at each tooth position (except for molars): the milk or deciduous teeth of the young and the permanent teeth of an adult mammal. Diphyodonty solves a functional-morphological dilemma: the size of teeth, an essential factor in feeding efficiency, is limited by the size of the jaws. While the jaws can grow extensively, the posteruption size of the teeth cannot be changed due to the rigidity of their enamel cover, which is the essential quality of a tooth. With diphyodonty, the size of the late erupting permanent teeth can be maximized and adapted to adult jaw size while the deciduous dentition provides a corresponding solution for the postweaning period. Dental morphology and the patterns of tooth replacement are specifically modified in some clades. In marsupials, only one milk tooth—the last premolar—comes in eruption, while the others are resorbed prior to eruption. Dolphins, aardvarks, and armadillos have a homodont dentition without any tooth replacement. No tooth replacement occurs in small and short-living mammals with greatly specialized dentition, such as shrews or muroid rodents (deciduous teeth are resorbed in-stead of eruption), while in some large herbivores tooth re-placement can become a continuous process by which the tooth row enlarges gradually by subsequent eruption of still larger molar teeth in the posterior part of the jaws. In elephants and manatees, this process includes a horizontal shift of the erupting tooth, which thus replaces the preceding cheek tooth. All these processes are well synchronized with the growth of jaws, the course of tooth wear, and subsequent pro-longing of time available for tooth development. (34) A general enlargement of the brain related perhaps not only to an increase in the amount of sensory information and/or a need to integrate sensory information from different sources, but also to more locomotory activity, high versatility in locomotory functions, a greatly diversified social life, and a considerably expanded role for social and individual learning. (38) The extended spectrum of behavioral reactions and their interconnections with an increased capacity of social and individual learning and interindividual discrimination should also be mentioned. In fact, this characteristic is very significant for mammals, as are the following two: (39) Growth is terminated both by hormonal control and structural factors. The most influential structural aspect of body growth is the appearance of cartilaginous epiphyseal discs separating diaphyses and epiphyses of long bones. With completed ossification, the discs disappear and growth is finished. Corresponding mechanisms determine the size of the skull (except in cetaceans, which have a telescoped skull in which the posterior bones of the cranium overlap each other). (40) Sex is determined by chromosomal constitution ()CYsystem, heterogametic sex is a male).
Almost all of these (and other) characteristics undergo significant variations and their modifications are often largely specific for particular clades of mammals. What is common for all is perhaps that in mammals all the characters are more densely interrelated than in other groups (except for birds). The morphological adaptations related to locomotion or feeding are often also integrated for social signaling, physiological regulation, or reproductive strategy, and often are controlled by quite distant and non-apparent factors. Thus, the excessive structures of ruminant artiodactyls, such as the horns of bovids and antlers of deer, are undoubtedly significant in social signaling, in courtship and display behavior, and frequently are discussed as excessive products of sexual selection. However, the proximate factor of these structures, the hereditary disposition for excessive production of mineralized bone tissue, can actually be selected rather by its much lessobvious effect in a female: her ability to produce a large, extremely precocial newborn with highly mineralized long bones that enable it to walk immediately after parturition. The female preference for the excessive state of the correlated characters in a male, his large body size and display qualities, possibly supported by social learning, supplement the mechanisms of the selection in quite a non-trivial way. Such a multi-layered arrangement of different factors included in a particular adaptation is indeed something very mammalian.
A paleontologist answers: The product of the earliest divergence of amniotes and index fossils of the Cenozoic Mammals are the only extant descendants of the synapsids—the first well-established group of amniotes, named after a rounded temporal opening behind the orbit bordered by the jugale and squamosum bones. Since the beginning of amniotes, evolution of synapsids proceeded separately from the other amniotes, which later diversified in particular reptile lineages including dinosaurs and birds. The first amniotes recorded from the middle Carboniferous (320 million years ago) were just synapsids and just this Glade predominated in the fossil record of the terrestrial vertebrates until the early Triassic. A large number of taxa appearing among early synapsids represented at least two different clades: Eupelycosauria and Caseasauria. The former included large carnivorous forms and the latter were generalized small- or medium-sized omnivores. Since the middle Permian (260 mya), another group of synapsids called Therapsida dominated the terrestrial record. In comparison with pelycosaurs, therapsids had much larger temporal openings, a single pair of large canines, and clear functional and shape differences between the anterior and the posterior teeth. Two lineages of that group, Dicynodontia and Cynodontia, survived the mass extinction at the Permian/Triassic boundary (248 mya).
Immediate ancestors of mammals are found among the cynodonts. Mammals are closely related to cynodont groups called tritylodontids and trithelodontids, which first appeared during the late Triassic. All three groups, including mammals, had additional cusps on posterior teeth, a well-developed ramus mandibulae, and a complete secondary palate. In some of them (Diarthrognathus), the jaw joint was formed both by the original articulation (articulare-quadraturn) and by the mammal-like process (dentary-squamosal). In the oldest true mammals, the former jaw articulation is abandoned and removed in the middle ear. These characters are the index diagnostic features of a mammal in the fossil record (no. 23, 26, 27 of the above list).
The oldest mammals, Sinoconodon, Adelobasileus, Kuehneotherium, or Morganucodon (about 200–225 million years old), were all very small, with long heterodont dentition and a triangular arrangement of molar cusps designed for shearing. They were most probably quite agile night creatures resembling today's insectivores. The relative brain volume in the earliest mammals was close to that found in extant insectivores and about three times higher than in cynodonts. Of course, they still differed from the modern mammals in many respects. The derived characters of modern mammals (as re-viewed in the preceding text) did not evolve together but were subsequently accumulated during the long history of synapsid evolution.
In contrast to the medium- to large-sized diurnal dinosaurs, birds, and other reptiles that had dominated the terrestrial habitats, the early mammals were quite small, nocturnal creatures. Nevertheless, since the Jurassic period they grew in greatly diversified groups and at least four lineages of that radiation survived the mass extinction at the Cretaceous/ Tertiary boundary (65 mya). Three of these groups, monotremes, marsupials, and placentals, are extant; the fourth group, multituberculates, survived until the end of Oligocene. Multituberculates resembled rodents in design of dentition (two pairs of prominent incisors separated from a series of cheek teeth by a toothless diastema), but their cheek teeth and skull morphology were quite different from those in any other groups of mammals.
radiation of mammals appeared at the beginning of Tertiary, in the Paleocene.
That radiation produced many groups that are now extinct (including nine extinct
orders) as well as almost all the orders of modern mammals. During the
Paleocene and Eocene, other groups occupied the niches of current mammalian
It is important to remember that the fossil record of mammals, including detailed pathways of evolutionary divergences and/or the stories of particular clades, is much more complete and rich in information than in any other group of vertebrates. This is due to the fact that the massive bones of mammals, and in particular their teeth, which provide most information on both the relationship and feeding adaptation of a taxon, are particularly well suited to be preserved in fossil deposits. Due to this factor, the fossil record of mammals is perhaps the most complete among the vertebrates. Also, during the late Cenozoic, Neogene, and Quaternary, the fossil record of some mammalian groups (such as rodents, insectivores, and ungulates) is so rich that the phylogeny of many clades can be traced in surprisingly great detail by the respective fossil record. For the same reason, some of these fossils (e.g., voles in the Qua-ternary period) are the most important terrestrial index fossils and are of key significance not only for local biostratigraphies and precise dating of the late Cenozoic deposits, but also for large-scale paleobiogeography and even for intercontinental correlations. The late Cenozoic period is characterized by gradually increasing effects of climatic oscillations, including repeated periods of cold and dry climate—glacials—followed by the evolution of grass and the treeless grassland country. Many clades of mammals responded to these changes and produced the extreme specialists in food resources of the glacial habitats, such as mammoths, woolly rhinos, lemmings, cave bears, and cave lions.
The most diversified animals There are about 4,600 species of mammals. This is a relatively small number compared to the 9,600 species of birds or 35,000 fish species and almost nothing in comparison to about 100,000 species of mollusks or some 10,000,000 species of crustaceans and insects. Even such groups as extant rep-tiles (with 6,000 species) and frogs (with about 5,200 species) are more diversified at the species level. Nevertheless, in diversity of body sizes, locomotory types, habitat adaptations, or feeding strategies, the mammals greatly exceed all that is common in other classes.
Only birds and arthropods may approach such variety. However, at least in diversity of body size, the mammals clearly surpass even them. The body mass of the largest extant terrestial mammal—the African elephant Loxodonta africana— with shoulder height of 11.5 ft (3.5 m), reaches to 6.6 tons (6,000 kg). The extinct rhinocerotid Baluchitherium was about 18 ft (5.5 m) and 20 tons (18,000 kg), respectively. The largest animal to ever appear—the blue whale (Balaenoptera musculus)—with up to 98 ft (30 m) in length, reaches 220 tons (200,000 kg). In contrast to dinosaurs or elesmobranchians, which also produced quite large forms, the aver-age mammal is a small animal the size of a rat, and the smallest mammals such as a pygmy white-toothed shrew (Suncus etruscus) or Kitti's hog-nosed bat (Craseonycteris thonglongyai) have a body length of just 1.2–1.6 in (3–4 cm) and weigh only 0.05-0.07 oz (1.5–2 g).
Mammals colonized almost all habitats and regions on the Earth. They now feed on flying insects hundreds of meters above the ground; jump through foliage in the canopy of a tropical forest; graze in lowland savannas and high mountain alpine meadows; hunt for fish under the ice cover of arctic seas; burrow the underground labyrinths to feed on diverse plant roots, bulbs, or insects; cruise the world's oceans, or dive there to depths of 1.8 mi (3 km) in the hunt for giant squid. Some even sit by a computer and write articles like this.
About 4,600 species of mammals are arranged in approximately 1,300 genera, 135 families, and 25 orders. Rodents with 1,820 species, 426 genera and 29 families are far the largest order, while in contrast, 8 orders include less than 10 species, and four of them are even monotypic (Microbiotheria, Notoryctemorphia, Tubulidentata, Dermoptera). Although inter-relationship among individual orders is still the subject of a vivid debate, three major clades of mammals are quite clear: monotremes (2 families, 3 genera, 3 species), marsupials (7 orders, 16 families, 78 genera and 280 spp.), and eutherian or placentals (17 orders, 117 families, 1,220 genera, 4,300 spp.), the latter two clades are together denoted as Theria.
The essential differences among the three major clades of mammals are in mode of their reproduction and patterns of embryonic development. Monotremes (platypus and echidnas), restricted to the Australian region, show only little difference from their ancestral anmiote conditions. They deliver eggs rich in yolk, and incubate them for 10 to 11 days. Young hatch from the egg in a manner similar to birds. Monotremes also retain the reptile conditions in the morphology of the re-productive system: the ovary is large and short oviducts come via paired uteri to a broad vagina, which opens with the urinary bladder and rectum into a common cloaca. Except for monotremes, all mammals are viviparous with intrauterine embryonic development and have quite small eggs, poor in yolk (particularly in eutherians).
There are essential differences between marsupials and eutherians in the earliest stages of embryonic development, as well as in many other characteristics. The reproductive tract in a female marsupial is bifurcated (with two vaginas), and also the tip of the penis in a male marsupial is bifurcated. Many marsupials have a marsupium, the abdominal pouchfor rearing young, supported with the marsupial epipubic bones that are present in both sexes. The marsupial intrauterine development is very short and the embryo is attached to the uterine endometrium by the choriovitelline (yolk) placenta that lacks the villi penetrating deeper in the wall of uterus (except in bandicoots). The marsupial new-borns are very small and little developed, and birth is non-traumatic. In contrast, the lactation period is much longer than in eutherians (only bats and some primates have proportionally long lactation periods). Nevertheless, the mother's total investment by the time of weaning young is roughly equal in both clades, but its distribution is different. The marsupial strategy is much less stressful for a mother and allows an extensive variation in tactics of reproduction. For instance, in the kangaroo, a mother can have three generations of young at one time: the young baby returning to drink low-protein but high-fat milk, the embryo-like young attached to a nipple nourished with high-protein but low-fat milk, and an embryo in the uterus for which development is delayed until the second-stage young is released.
A key agent of eutherian reproduction is the highly specialized organ supporting a prolonged embryonic development—the chorioallantoic placenta. Eutherian newborns are large and despite considerable variation over particular clades, are potentially capable of an independent life soon after birth. Large herbivores such as elephants, perissodactyls, and artiodactyls, as well as cetaceans, sirenians, hyraxes, and some primates, deliver single, fully developed newborns with open eyes, ears, and even the ability to walk immediately after birth. Such a newborn is called precocial in contrast to the altricial newborns of insectivores, bats, rodents, or carnivores, which are hairless, blind, and fully dependent on intensive mother's care. Both developmental strategies may, of course, appear within one Glade as in lagomorphs (large litters and altricial young in a rabbit versus small litters and precocial young in a hare). Variations in reproductive strategies are closely interconnected with numerous behavioral adaptations and adaptations in social organization and population dynamics, all of which contribute significantly to mammalian diversity.
Recent molecular data strongly support the essential role of geographic factors in phylogenetic history and in taxonomic diversity of mammals. Thus, there is very strong sup-port for the African clade Afrotheria, which is composed of the tenrecid and potamogalid insectivores, golden moles, macroscelids, aardvark, hyraxes, proboscideans, and sirenia. Also, the extensive covergences between Australian marsupials and particular eutherian clades and/or the paleontological data on mammalian evolution on particular continents suggest that on each continent, the adaptive radiation produced quite similar life forms: small to medium sized insectivores, rodent-like herbivores, large herbivores, and their predators. The niche of large herbivores seems to be particularly attractive (at least 18 different clades attained it) but at the same time, it is perhaps the most dangerous (13 of them are extinct).
fourth of all mammals fly. This is pertinent to a number of species, the number
of genera, and perhaps for the number of individuals as well. Bats, with more
than 1,000 species in 265 genera, are the most common mammals in many tropical
and subtropical habitats. Mostly active at night, bats hunt for various kinds of
aerial prey (a basic strategy of the clade) or feed on fruit, nectar, or pollen.
Some bats feed on frogs, reptiles, or other bats, and in the tropics of
However, most of the extant mammals (nearly a half of all genera) maintain the basic mammalian niche. They are terrestrial, mostly nocturnal or crepuscular, and forage for different food resources that are available on the ground. In a tropical forest this may be seeds and fruits falling down from the canopy and the invertebrate or vertebrate animals feeding on them. In the subtropics and temperate regions, the significance of this habitat increases as the soil surface becomes the most significant crossroads of ecosystem metabolism. In a temperate ecosystem, the soil is the major conveyer of the energetic flow and an important source of freeenergy that is available in a variety of food resources. It is no wonder that in the temperate regions terrestrial mammals form more than half of the local mammalian taxa (while it is one third or less in the tropics) and that their densities exceed those of all remaining mammalian species. Among them we find the groups that are the most progressive and most rapidly diversifying clades of the extant mammals (such as shrews or muroid rodents). Terrestrial mammals are, as a rule, quite small animals, and are often the r-strategists. They have short life spans, large litter sizes, several litters per year, and rapidly attain sexual maturity, sometimes even a few weeks after birth. Most of the small ground mammals dig underground burrows for resting. This reduces not only the risk of predation, but due to stable microclimatic conditions of the underground habitat, it also reduces metabolic stress by ambient temperature or by daytime changes in other weather conditions. Many mammals also tend to spend a consider-able part of their active life underground, including food gathering. Those that combine it with terrestrial foraging are called semifossorial—most of the 57 genera of semifossorial mammals are rodents. Those that are entirely adapted to an underground way of life and often do not come above ground at all are called fossorial. The fossorial adaptations, which make them all quite similar in general appearance, are seen in 35 genera of 13 different clades and evolved convergently in all major geographic regions (Australian marsupial mole, Holoarctic true moles, the African golden moles, and 10 groups of rodents in Holoarctic, Ethiopian, and Neotropical regions). Compared to their relatives, the fossorial mammals are all the K-strategists, some with pronounced tendencies to complex organization (mole rats).
also evolved another way to inhabit terrestrial habitats. It is called
scansorial adaptation and is typical of large herbivores with an enormous
locomotory capacity, enabling them to exploit distant patches of optimal
resources and react actively to seasonal changes in them. In many in-stances
these are social animals living in large nomadic herds. Kangaroos, the large
macropodid marsupials of
Locomotory modes are entirely different in the 156 genera of mammals that forage in arboreal habitats. Essentially arboricolous are primates, dermopterans, and tree shrews, as well as many marsupials, rodents, bats, and some edentates and carnivores. Typical for most of them are long forelimbs and a long tail, often prehensile. Other arboricolous mammals have a haired membrane between their legs, enabling them to glide between tree trunks. The mammals equipped for such gliding flight include flying lemurs (Dermoptera), several groups of rodents (flying squirels, African anomalurids), and three genera of marsupials.
Roughly 107 genera and 170 species are aquatic or semi-aquatic and mostly fish-eating. Three grades can be distinguished here: (1) terrestrial animals that enter aquatic habitats only temporarily for feeding only (African otter shrews, Old World water shrews, desmans, water opossum, more clades of rodents, including large rodents such as beaver and capybara, and several clades of carnivores, particularly otters); (2) marine mammals that spend most of their life in aquatic habitats but come to shore for breeding (all pinnipedian carnivores, such as seals, sea lions and walruses, and sea otters); and (3) the exclusively aquatic mammals incapable of surviving outside of the aquatic environment—sirenians and cetaceans. The latter group is quite diversified, and includes 78 species in 41 genera that can be subdivided into two major clades: Mysticeti, whales that filter marine plankton with baleen plates hanging from roof of the mouth cavity, and Odontoceti, dolphins and toothed whales, which echolocate and feed on fish or squid (including the giant deep-sea architeuthids as in the sperm whale). Cetaceans evolved various sophisticated adapatations for prolonged diving into deep oceanic waters, such very economic ways of gas exchange that include a reduced heart rate during diving and more oxygen-binding hemoglobin and myoglobin in blood than in other mammals. Cetaceans, though closely related to non-ruminant artiodactyls and recently included together with them in a common order, Cetartiodactyla, diverge from the common picture of "what is a mammal?" perhaps most of all.
The extreme diversity in feeding adaptations is among the most prominent characteristics of mammals. Feeding specializations such as grazing grass or herbal foliage, palynovory (eating pollen of plants), myrmecophagy (specialized feeding on ants and termites), and sanguivory (feeding on blood ofbirds and mammals, in five species of true vampires) are no known from any other vertebrates. At the same time, all the feeding adaptations occurring in other vertebrate clades occur also among mammals.
In all mammals, the efficiency of a feeding specialization depends upon the appropriate morphological, physiological, and behavioral adaptations. First, it concerns the design of the teeth and dentition. The generalized heterodont dentition and the tribosphenic molar teeth designed for an insectivorous diet (as retained in various marsupials, insectivores, tree shrews, prosimian primates, and bats) can be easily modified to the carnivorous diet. A carnivorous diet further demands enlarging the size of the canines and arrangements that increase the shearing effect of cheek teeth. A lower position of the jaw joint increases the powered action of temporal muscles at the anterior part of dentition, and in extremely specialized carnivores such as cats, the dentition is then considerably shortened and reduced except for canines and the carnasial cheek teeth (the last upper premolar and the first lower molar, generally the largest teeth of carnivores). There is no problem with digesting the tissues of vertebrates and thus no special arrangements of the alimentary tract are needed.
In contrast, herbivores, especially those specialized in feeding on green plant mass, require a modified jaw design. This kind of food is everywhere and easily accessible as a rule, but it is extremely difficult to digest for several reasons. One is that this diet is very poor in nutritive content and must be consumed in very large volumes; it must also be broken down mechanically into small particles. Hence, the dentition is overburdened by wear of occluding teeth and their abrasion with hard plant tissue. Efficiency of feeding depends directly on the design of the tooth crown, on the size of total area for effective occlusion, and the efficiency of masticatory action. Large teeth with flat surfaces and high crowns resistant to intensive wear are particularly required.
The major problem with a diet of plants is that mammals (as well as other animals) do not produce enzymes that break down cellulose. They must rely on symbiotic microorganisms residing in their alimentary tract, evolve an appropriate housing for them, and ensure a sufficient time for proper food fermentation. The mammals evolved several ways to fulfill these requirements. One is the foregut fermentation (digastric digestion system) characteristic of ruminant artiodactyls (bovids, cervids), kangaroos, and colobus monkeys. The fermentation chambers are situated in spacious folds of the stomach; from these fermentation chambers the partially fermented food can be regurgitated and chewed during a rest period, which also prolongs the movement of food through the gut. The microorganisms detoxify alkaloids by which growing plants de-fend against herbivores prior to digestion, but are very sensitive to tanins contained in the dry plant tissues. The foregut fermenters avoid dry plants but feed on growing parts of plants, selectively cut with the tongue and lips (ruminants even lack the upper incisors).
Perissodactyls, rodents, lagomorphs, hyraxes, and elephants evolved hindgut fermentation (monogastric digestion system), where fermenting microorganisms are housed in the caecum and large intestine. Food is not regurgitated and all mechanical disintegration of food must be performed at one mastication event. Except for caeca, the passage of food through the gut is almost twice as fast as in the foregut fermenters. Hindgut fermenters can survive on a very low-quality food, if it is avail-able in large quantity. They can effectively separate the tanins and dry plant mass, both of which decrease the efficiency of the foregut fermenting. Correspondingly, the foregut and hindgut fermenters prefer different parts of plants and can both forage in the same habitats without any actual competition. The latter are, of course, under more intense pressure to evolve further adaptations to compensate for the energetic disadvantages of their digestion. One of them is extreme enlargement of caeca (as in rodents); another is considerable increase in the height of cheek teeth (maximized in several clades of lagomorphs and rodents, in which cheek teeth are hypselodont, or permanently growing). The third way is an increase in body size. This enlarges the length of the alimentary tract and prolongs the passage of food through it, while at the same time it reduces the rate of metabolism. The behavioral reduction of metabolic rate by a general decrease of activity level as in foliovore (leaf-eating) sloths or the koala produces the same results.
The gradual increase in body size is a feature of mammalian evolutionary dynamics, as it was repeatedly demonstrated by the fossil record of many clades. This is seen in most eutherians (not only in the herbivorous clades), but is much less apparent in marsupials. It seems that in addition to the common factors promoting a larger body size (a reduced basal metabolic rate, smaller ratio of surface area to body mass, and smaller heat transfer with ambient environment), something else comes into play, something which has to do with the essential differences of both the clades. This is the enormous stress of the eutherian way of reproduction. While intrauterine development is short and a litter weight is less than 1% of the mother body mass in a marsupial, the eutherian female must endure a very long pregnancy and the traumatic birth of a litter that in small eutherians such as insectivores, rodents, or bats, may weigh 50% of the mother's body mass.
With enlarging body size, the stress of pregnancy and parturition is reduced as the size of a newborn is relatively smaller (compared with 3-5% of a mother's mass in large mammals and 10-20% in smaller mammals). With a reduction of litter size, it further provides a chance to refine the female investment and deliver fully developed precocial young, as in ungulates or cetaceans. This aspect of mammalian adaptation and diversity should remind us that perhaps the ways in which a female does manage the stress of eutherian reproduction (the factor that magnified the strength of selection pressure) became the most influential source of viability of our clade.
Neighbors, competitors, and friends
Mammals and humans have been the closest relatives and nearest neighbors throughout the entire history of humankind. Mammals contribute essentially to our diet and wekeep billions of domesticated mammals solely for that purpose. Hunting mammals for protein-rich meat became an essential background factor in human evolution several million years ago. More recently, the discovery of how to get such animal protein in another way started the Neolithic revolution some 10,000 years ago. The symbiotic coexistence with herds of large herbivores—which included taking part in their reproduction and consuming their milk and offspring—ensured the energetic base for a considerable increase in the human population of that time and became one of the most important developments in human history. Moreover, the other essential component of the Neolithic revolution may be related to mammals. Feeding on seeds of grass and storing them in the form of a seasonal food reserve could hardly have been discovered without inspiration from the steppe harvesting mouse (Mus spicilegus) and its huge corn stores or kurgans, containing up to 110 lb (50 kg) of corn. The theory that humans borrowed the idea of grain storage from a mouse is supported by the fact that the storage pits of Neolithic people were exact copies of the mouse kurgans. Mammals have even been engaged in the industrial and technological revolutions. Prior to the steam engine and for a long time in parallel with it, draft animals such as oxen, donkeys, and horses were a predominant source of power not only for agriculture, transport, and trade, but also for mining and early industry. Indeed, our civilization arose on the backs of an endless row of draft mammals.
At the same time, many wild mammals have been considered dangerous enemies of humans: predators, sources of epizootic infections, or competitors for the prey monopolized by humans. Many mammals were killed for these reasons, while some were killed merely because we could kill them. As a result, many species of wild mammal were drastically reduced in numbers leading to their local or global extinctions. The case of the giant sea cow (Hydrodamalis stelleri) is particularly illustrative here, but the situation with many other large mammals, including whales, is not much different. The introduction of cats, rats, rabbits, and other commensal species to regions colonized by humans has badly impacted the native fauna many times, and the industrial pollution and other impacts of recent economic activity act in a similar way on a global scale. About 20% of extant mammalian species may be endangered by extinction, mostly due to the destruction of tropical forest.
However, since the Paleolithic, humans also have kept mammals as pets and companions. Even now, the small carnivores or rodents that share our houses bring us a great deal of pleasure from physical and mental contact with something that, despite its apparent differences, can communicate with us and provide what often is not available from our human neighbors—spontaneous interest and heartfelt love. Contact with a pet mammal may remind us of something that is al-most forgotten in the modern age: that humans are not the exclusive inhabitants of this planet, and that learning from the animals may teach us something essential about the true nature of the world and the deep nature of human beings as well.
Volume 17. Cumulative Index edited by Melissa C. McDade (Gale Group) A useful tool for the whole set. It is especially helpful when one has no idea where to find a particular animal. Scorpions for example are found in volume two, not three.
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