Most amphibians begin their lives in the water as tadpoles, or larvae, which breathe by means of external gills instead of lungs. At first the tadpole has no definite shape, and no tail can be seen. The mouth is a V-shaped sucker on the underside of the body. As the head grows, a round mouth with a horny rim develops. At the same time, the tadpole grows a flat, finlike tail. The tiny creature later changes to adult form and breathes at least partly through lungs. This transformation process is called metamorphosis (from the Greek meta, meaning "change," and morphe, meaning "form"). The larval stage lasts from several weeks to one year, depending on the particular species and upon environmental factors such as temperature and humidity. Certain species of amphibians, particularly among the salamanders, remain in larval form all their lives. This phenomenon is called neoteny.
The larvae of the three orders differ from one another in several ways. The urodele larvae are long and slender, with limbs, three pairs of gills, and large mouths. The anurans, with short trunks and small mouth openings, lack lungs, eyelids, jaws, and legs. They look much more like fishes than like frogs or toads. The gymnophion larvae are limbless and slender and have distinctive gills.
Although a female amphibian is capable of producing thousands of offspring during the course of a lifetime, the general population of amphibians remains about the same. This is because during the process of metamorphosis many die and others are destroyed by predators.
Metamorphosis alters the feeding and breathing habits of an animal as well as its physical shape. Amphibians change from gill breathers to lung breathers. They also change from plant eaters to meat eaters. Adults eat insects or small animals, especially mollusks, worms, and other amphibians. Some frogs also eat small mammals and birds.
Most amphibians reach maturity at three or four years. They breed for the first time about one year after metamorphosis.
Although some amphibians carry their eggs within their bodies until hatching takes place, most lay eggs in moist places to allow their offspring to develop at least to the larval stage before hatching. The eggs vary from 1/25 of an inch (1 millimeter) to 1/6 of an inch (4 millimeters) in diameter, depending on the species. Some species abandon their eggs in a pool, a stream, or some other moist place, but others watch over their eggs until they hatch.
The life span of an amphibian depends on its species and on its environment. Amphibians in captivity have been known to live up to 20 years. The general life span probably ranges from 10 to 40 years.
Main Page - Kinds of Amphibians - Physical Characteristics - Behavior
The Evolutionary Record - Anatomy of the Frog