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The vertebrate class Amphibia, to date, includes about 3,500 species in three orders: frogs and toads (order Anura), salamanders and newts (order Caudata), and caecilians (order Gymnophiona). There is, however, a much larger number of extinct species, because this ancient group of animals were the first vertebrates to begin exploiting terrestrial environments. Fossil amphibians are known from at least the Devonian era, about 400 million years ago. However, this group was most diverse during the late Carboniferous and Triassic eras, about 360-230 million years ago.

None of the surviving groups of amphibians can be traced back farther than about 200 million years. All of the living amphibians are predators as adults, mostly eating a wide variety of invertebrates, although the largest frogs and toads can also eat small mammals, birds, fish, and other amphibians. In contrast with adults, larval frogs and toads (tadpoles) are mostly herbivorous, feeding on algae, rotting or soft tissues of higher plants, and infusions of microorganisms.

Amphibians are poikilothermic animals—their body temperature is not regulated, so it conforms to the environmental temperature. Amphibians have a moist, glandular, scaleless skin, which is poorly waterproofed in most species; this skin allows gaseous exchange and actively pumps salts. Most amphibians have tails, but the tail in adult frogs and toads is vestigial, and is fused with the pelvis and sacral vertebrae into a specialized structure called a urostyle. Some species of caecilians have lost their limbs and limb-girdles, and have a wormlike appearance.

All amphibians have a complex life cycle, which begins with eggs that hatch into larvae, and eventually metamorphose into adult animals. Usually, the eggs are laid into water and are externally fertilized. The larvae or tadpoles have gills or gill slits and are aquatic. Adult amphibians may be either terrestrial or aquatic, and breathe either through their skin (when in water) or by their simple saclike lungs (when on land). However, these are all generalized characteristics of the amphibian lifestyle; some species have more specialized life histories, and can display attributes that differ substantially from those described above. Rare idiosyncrasies of amphibian life history can include ovoviviparity, in which fully formed, self-nourishing, developing eggs are retained inside the female's body until they hatch as tadpoles, and even viviparity, in which larvae develop within the female but are nourished by the parent, as well as by their incompletely formed egg, until they are released as miniature frogs.

Frogs and toads lack tails but have greatly enlarged hind legs that are well adapted for jumping and swimming. Most of the living species of amphibians are anurans, comprising about 3,000 species. Most anurans are aquatic, but some are well adapted to drier habitats. Some common anurans of North America include the bullfrog (Rana catesbeiana, family Ranidae), spring peeper (Hyla crucifer, family Hylidae), and the American toad (Scaphiopus holbrooki, family Pelobatidae). The latter species lives in arid regions, estivating (spending the summer in hibernation) during dry periods but emerging after rains to feed, and taking advantage of heavy but unpredictable periods of rain to engage in frenzies of breeding. The largest frogs reach 11.8 in (30 cm) in length and weigh several pounds.

There are about 250 species of newts and salamanders, ranging in size from approximately 6 in (15 cm) to more than 5 ft (1.5 m). These amphibians have a tail and similarly sized legs well adapted to walking, but are usually found in or near water. Most species lay their eggs in water, however, adults usually spend most of their time in moist habitats on land. An exception is the red-spotted newt (Notophthalmus viridescens, family Salamandridae) of eastern North America, which in its juvenile stage (the red eft) wanders in moist terrestrial habitats for several years before returning to water to develop into its aquatic adult stage. Some species, such as the The life cycle of a frog. Illustration by Hans & Cassidy. Courtesy of Gale Group.

lungless red-backed salamander (Plethodon cinereus, family Plethodontidae) of North America, are fully terrestrial. This species lays its eggs in moist places on the forest floor, where the animals develop through embryonic and larval stages and hatch directly as tiny versions of the adult stage.

Caecilians are legless, almost tailless, wormlike, burrowing amphibians found in moist, tropical habitats. They feed on soil invertebrates. There are at least 160 species of caecilians, reaching 5 ft (1.5 m) in length, but most are rarely seen despite their size.

Recently, within the last 50 years, an alarming global decline in the gross number of amphibians and amphibian species has been documented. Because of their dependence on fresh water supplies for reproduction, and since almost all species spend their adult lives in moist environments, it is hypothesized that widespread water pollution is catalyzing the decline of this class of organisms. In other cases, the extinction and decline of species is known to be the direct consequence of human intervention (habitat destruction). However, scientists have noted that in untouched, pristine environments where pollution and human encroachment are minimal or nonexistent, amphibian populations also are declining. As such, it is speculated that other changes, such as increased UV radiation due to ozone depletion, or the introduction of competing exotic species is responsible for their collapse. The decline is unfortunate because amphibians are important indicators of the health of ecosystems, sources of potent and medically useful chemicals, stabilizers of ecological balance in areas where they live, and contributors to the aesthetic beauty of the natural world.

See also Chordates.

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