Amphibians are perhaps the most unique group of vertebrates on earth. There are over 5,000 species living in very diverse habitats.
Amphibian literally means "double life," and are named so because most species require both land and water to carry out their lifecycle. In most species, adults migrate to the water where they mate and lay eggs. The gelatinous eggs then hatch and turn into larvae (tadpoles, if frogs or toads). Larvae live in the aquatic environment for various lengths of time, and undergo metamorphosis. At this time, gills disappear and lungs are formed and the tail is resorbed into their body and they become juveniles. While this phenomenon is not unique to amphibians (insects do it too), it is quite unique among vertebrates. Occasionally, amphibian species such as the tiger salamander, will undergo metamorphosis and become sexually mature, but still retain the characteristics of a larva in a process called paedomorphosis. This allows these organisms to fully exploit an aquatic habitat when the terrestrial one may be less sustainable.
Some species of salamanders do not have lungs. While all amphibians have the ability to exchange gases (breathe) through their skin, plethodontid salamanders rely on this as their primary method of getting oxygen. Some species of amphibians retain their gills throughout life, and are restricted to aquatic habitats.
Male amphibians make calls to attract females. Females do not call. Some frog calls, like that of the spring peeper, can reach intensities of up to 90 decibels and can be heard from miles away. Males call most intensely after rains, to assure that the eggs they fertilize will have adequate water in which to develop. Most species of amphibians do not exhibit parental care. So, when they lay their eggs they do not care for their young. Some species of poison dart frogs, however, will allow their tadpoles to wiggle onto the back of the male frog and be carried about. There are also species that build foam nests in the tree branches overhanging ponds. When the tadpoles hatch they fall into the water below and are at that time large enough to avoid many more predators had they hatched in the pond.
Amphibians live in varying habitats ranging from the tundra in central Alaska, to the deserts of North America and Africa. Wood frogs, which occur in the far north have a molecule in their bloodstream, similar to an antifreeze, that allows them to freeze solid. When temperatures warm again they thaw. Desert-dwelling toads have the ability to burrow many feet into the ground to conserve moisture and avoid the heat. When the rains beat against the soil, they burrow upwards, find a mate and head toward a puddle to lay their eggs. Some of these species' tadpoles will develop into small toads within 8 days of being deposited as eggs.
Amphibian diets are varied. Some species are opportunistic and will eat nearly anything. I have witnessed a bullfrog eat a baby duck. Other species are very specialized. Some species of toads eat mostly ants.
Because amphibian physiology is so closely tied to their environment, they make excellent environmental indicators. I conduct research on the effects of contaminants on amphibians, and know first hand that they are sensitive. Amphibian populations have been declining worldwide for a number of years. Humans impact their environment through pollution and modifying their habitat, making it unliveable.