What is an amphibian?

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 world.

Defining characteristics

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.

Systematics (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 Madagascar has been recognized as a family, Mantellidae, a subfamily of Ranidae, and a subfamily of Rhacophoridae (adopted herein). The classification used in this volume is order Gymnophiona (caecilians) with five families, order Caudata (salamanders) with 10 families, and order Anura (frogs and toads) with 28 families.

Historical biogeography

The 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 North America, Europe, and most of Asia; and Gondwana, which included what are now South America, Africa, Madagascar, the Indian subcontinent, Australia, New Zealand, and Antarctica. Prototypic lissamphibians apparently were rather widely distributed in Pangaea before the continental fragmentation.

Although a 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 Asia), one family is endemic to Africa, and another to South America.

Salamanders evolved in Laurasia. One family is restricted to Asia, and four families are shared by Eurasia and North America, where five families are endemic. Only one lineage (Plethodontidae) has dispersed from North America to South America.

The biogeography of anurans is somewhat more complicated. One early lineage containing the living Ascaphidae in North America and Leiopelmatidae in New Zealand has been allied with fossils from the Jurassic of South America, thereby

indicating that this lineage had diversified prior to the breakup of Pangaea. The fossil records and present distributions of other lineages of archaeobatrachians (primitive frogs) are in Laurasian continents: Bombinatoridae, Discoglossidae, Megophryidae, Pelodytidae, and the fossil Paleobatrachidae in Eurasia; Pelobatidae in Eurasia and North America; and Rhinophrynidae in North America. However, the historical biogeography of most anurans (neobatrachians or advanced frogs) is associated with Gondwana, the fragmentation of which into the existing continents played a major role in the differentiation of many lineages of anurans. Many lineages are restricted to one continent: six families in South America, three in Africa, two in Australia, and one each in Madagascar and the Seychelles. Others are shared with two or more Gondwanan continents: one (Pipidae) in Africa and South America; one (Hylidae) in South America and Australia (also via dispersal into North America and Eurasia); one (Hyperoliidae) in Africa, Madagascar, and the Seychelles; and another (Rhacophoridae) in those three regions plus the Indian subcontinent and adjacent southeastern Asia. Microhylidae is present on all Gondwanan land masses except the Seychelles, and it has dispersed into southeastern Asia and southern North America. True frogs (Ranidae) occur throughout the world, though only in northern Australia and northern South America

on these continents, and toads (Bufonidae) occur on all continents, except Australia (one species introduced).

Regional diversity

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.

Globally, except for the Arctic and Antarctic regions (which are not inhabited by amphibians), six biogeographic regions are recognized. The largest of these, the Palearctic (Europe and temperate Asia) has the fewest species of amphibians (192), followed by the Nearctic (temperate North America) with 243 species. Historically, these regions are part of the former Laurasia and have the greatest diversity of salamanders, especially in the Nearctic. In contrast, the amphibian faunas of the southern continents consist mainly of anurans. The Australo-Papuan region (Australia, New Zealand, New Guinea, and associated islands) has 450 species of anurans, but no salamanders or caecilians. The Ethiopian or Afrotropical region (sub-Saharan Africa and Madagascar) has 770 species, of which 29 are caecilians. The Oriental region (tropical and subtropical southeastern Asia, India, and associated islands harbors 825 species, of which 29 are salamanders and 44 are caecilians. By far the greatest amphibian diversity is in the Neotropical region (South America, tropical Mesoamerica, and the West Indies) with 82 species of caecilians, about 200 species of salamanders, and more than 2,500 species of anurans.

Although caecilians are pantropical, they are absent in Madagascar. Ichthyophiidae and Uraeotyphlidae are endemic to the Oriental region, Scolecomorphidae to the Ethiopian region, and Rhinatrematidae to the Neotropical region. The large family Caeciliidae is most diverse in the Neotropical region (14 genera and 73 species) and is present in Africa (6 genera and 17 species), Oriental region (2 genera and 4 species), and in the Seychelles Islands in the Indian Ocean (3 genera and 7 species).

Most 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 Europe. However, the greatest diversity of plethodontids is in tropical America, especially Central America and southern Mexico, where 12 genera with about 200 species occur; two of these genera also occur in South America, one as far south as Bolivia.

Only four of the 28 families of anurans occur in both the Old and New Worlds. Bufonidae is global in its distribution, except for Australia, New Zealand, and Madagascar. Ranidae has a similar pattern, but also occurs in Madagascar and in northern Australia. Microhylidae has a few representatives in the Nearctic and Palearctic regions and is highly diverse on the southern continents, including Madagascar and New Guinea, but not in New Zealand. Hylidae is most diverse in the Neotropical region and secondarily in the Australo-Papuan region. Two genera are endemic to North America, and only a few species of Hyla inhabit the Oriental and Palearctic regions.

With the exception of Pelobatidae in the Nearctic and Palearctic regions, all other families of anurans are restricted to the New World or the Old World, and only a few of these are in the Northern Hemisphere. Ascaphidae is endemic to the Nearctic region, Megophryidae to the Oriental region, Discoglossidae and Pelodytidae to the Palearctic region, and Bombinatoridae in the Palearctic and Oriental regions. The greatest diversity is in the southern land masses. Leiopelmatidae is endemic to New Zealand, and Limnodynastidae and

Myobatrachidae are endemic to the Australo-Papuan region. The Ethiopian region has six endemic families of anurans: Arthroleptidae, Heleophrynidae, Hemisotidae, Hyperoliidae (Africa, Madagascar, and Seychelles), Scaphiophrynidae (Madagascar only), and Sooglossidae (Seychelles only). The greatest diversity of Rhacophoridae is in the Oriental region, but the family also is diverse in Madagascar and has one genus with three species in Africa.

The 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 Africa) and Ranidae (shared with much of the world).

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 earthworms (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.

In summary, 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.

Resources

Books

Duellman, William E., ed. Patterns of Distribution of Amphibians. Baltimore: Johns Hopkins University Press, 1999.

Duellman, William E., and Linda Trueb. Biology of Amphibians. Baltimore: Johns Hopkins University Press, 1994.

Zug, George R., Laurie J. Vitt, and Janalee P. Caldwell. Herpetology. 2nd ed. San Diego: Academic Press, 2001.

William E. Duellman, PhD