Amphibians – frogs, toads, salamanders, and caecilians – were the first vertebrates to leave the water for a terrestrial existence. During the Devonian Era, some 360 million years ago, they arose from a fish-like ancestor that developed lungs and moved about on muscular fins. They form the evolutionary link between fish, reptiles and birds.
Amphibians are divided into the salamanders, all of which have tails, some 400 species; frogs and toads, which are tailless, some 400 species; and the Caecilians, which have tails but no legs, 155 species, only one of which, the “rubber eel” appears in the pet trade.
Bound by their porous skin to moist habitats, salamanders reach their greatest numbers and diversity in temperate habitats. Frogs, which exist worldwide, are most common in the tropics. The wood frog, however, reaches the Arctic Circle and some frogs have adapted even to living in the world’s driest deserts. The caecilians are pan-tropic in distribution.
Amphibians range in size from a Brazilian frog that, at 1½ inches long, is the world’s smallest four-legged animal to the 145-pound Chinese giant salamander, the largest amphibian, which approaches 6 feet in length.
What most distinguishes amphibians is the fact that they begin their lives in the water and then metamorphose into land creatures that lay their eggs in the water. When the eggs hatch, the larvae live aquatic lives as they grow, sometimes for as much as a year, before they become terrestrial adults. The most familiar amphibian larvae is the tadpole, which swims about in ponds until it grows legs, loses its tail and matures into a frog. All amphibians go through this same process and larvae hatched in aquariums can be observed through their metamorphosis.
Amphibians are ectothermic, which means that once they’re out on land their body temperature is dependent upon the temperature outside. Since most amphibians can live in cooler temperatures than most reptiles, providing heat sources for basking is not usually necessary.
Water is necessary and is absorbed by the amphibian’s skin. The skin is also used (in some amphibians exclusively) for respiration. The amphibians have eyes that provide for good depth perception and color vision. Hearing is important for vocal frogs, as anyone who’s heard the intensity of their croaking can attest. Salamanders have a keen sense of smell, using it and their quick and agile movements to find and capture prey. Their teeth are often reduced, food is swallowed whole, and the long, and sometimes sticky, tongue may be projected out to catch prey
While the aquatic larvae feed, like fish, on small crustaceans, adult amphibians eat a wide variety of foods, which makes them easy to feed in captivity. Frogs and toads feed mostly on insects, although large species will eat small birds and mammals. Salamanders also feed on insects but will eat small worms and insect larvae. Aquatic species will eat fish.