Earth Science: Vertebrates – Amphibians
Amphibians are descendants of the first vertebrates to move out of water onto land. Their lives still reflect that heritage: they spend their early life (larval phase) in water and the rest on land.
Amphibians include both tailed animals and tailless species and can be divided into two groups: Caudata and Anura. The Caudata group, which includes newts and salamanders, has over 400 species living in the Northern Hemisphere and the American tropics.
Animals of this group have four legs, elongated bodies, a long tail, and often large eyes. Frogs and toads be- long to the Anura group and are native everywhere outside the polar regions.
They usually have powerful hind legs for hopping and jumping. Tree frogs have sticky toe pads, making them exceptional climbers.
Between water and land
Amphibians show their kinship to the first bony fish that used strengthened fins to climb onto land by beginning their lives as finned swimmers with gills. However, by adulthood they are air-breathing creatures with legs and are fully adapted to life on land. They are cold-blooded, so assume the surrounding temperature.
Many spend winters snuggled into holes in the ground or protected by piles of leaves. Their skin is highly sensitive and must not dry out, as it supplements their lungs by absorbing both air and moisture. Some amphibians have skins with glands that secrete toxic or bad-tasting fluids to protect them from predators.
Why frogs croak
Frogs have an acute sense of hearing and strong voices. Male frogs have developed mating calls (croaks) that female frogs find alluring. Salamanders, on the other hand, are quiet and attract partners with special scents and bright colors. Amphibians can have highly complex mating rituals.
Male pond frogs, for instance, release sperm to fertilize eggs laid as a cluster in the water. Female crested newts take up a packet of sperm for internal fertilization. Some amphibian clusters can contain over 10,000 eggs, whereas sometimes, only one egg is laid.
In rare cases amphibian species even bear live young. Only a few amphibians breed on land; these lay eggs in rotting leaves or tree cavities.
THE AMPHIBIAN LIFE CYCLE
Spring is mating time for amphibians and egg-laying follows, usually in a pond. After emerging from the eggs, the larvae metamorphose into adults, often within a few weeks. Frogs undergo a particularly dramatic change from tad-poles to adults.
They lose their tails and gills and develop powerful legs. Tadpoles are primarily vegetarians, but adult frogs eat insects and other animals. Salamanders feed on in-sect larvae and remain carnivorous throughout their lives.
BASICS
AMPHIBIAN NUMBERS are declining. Weedkillers and insecticides kill their food sources and are also absorbed through their skins Others die on roads as they travel (often several miles) between their habitats.