Amphibians are cold-blooded animals that possess backbones and display features that lie between those of fish and reptiles. They spend time both in water and on land. Their larvae (not yet fully developed offspring) mature in water and breathe through gills, like fish, while adults breathe air through lungs and skin. Amphibians are in the class Amphibia, which includes over 3,500 species. They are further divided into three orders: Anura (frogs and toads), Urodela (salamanders and newts), and Gymnophiona (caecilians, pronounced sih-SILL-yuhns, which are wormlike in appearance).
Amphibians evolved from fish about 400 million years ago, when the amount of dry land on Earth increased greatly. Certain fish adapted to these changing conditions by gradually developing limbs to crawl with and lungs to breathe with. Such organisms, capable of life both in water and on land, came to be called amphibians, a name that means "double life." Amphibians were the first vertebrates (animals with backbones) to live on land. However, they returned to the water to breed. The largest variety of amphibians occurred about 360 to 230 million years ago, when the environment was continually alternating between wet and dry conditions. Many of the species that developed during this period no longer exist. The groups of amphibians that survived to the present day can be traced back no further than 200 million years.
Amphibians are cold-blooded animals, meaning they do not have a constant body temperature but instead take on the temperature of their environment. They have moist, scaleless skin that absorbs water and oxygen, but that also makes them vulnerable to dehydration (loss of bodily fluids). Without moist conditions, their skin dries out and they die. Therefore, amphibians are most often found near ponds, marshlands, swamps, and other areas where freshwater is available. Some amphibians become inactive when conditions are unfavorable for survival. This period of inactivity is called estivation when it occurs during hot, dry weather and hibernation when it occurs in response to cold temperatures. Activity resumes when favorable conditions return.
The thin skin of amphibians contains many glands, among them poison glands that protect certain species against predators. The poison from the glands of the brightly colored poison-dart frog is particularly toxic and is used by South American Indians to coat the tips of their arrows. Some amphibians protect themselves from enemies by changing color to blend in with their surroundings.
The life cycle of most amphibians begins in water when the female lays eggs that are fertilized outside of her body. The eggs then hatch into larvae, or tadpoles, that breathe through external gills. The larvae grow flat tails and feed on vegetation. During a process called metamorphosis, physical changes occur and external gills give way to lungs. The tadpoles also change from plant-eating animals to meat eaters. Amphibians usually reach full adulthood at three to four years.
Estivation: State of inactivity during the hot, dry months of summer.
Gill: A bodily organ capable of obtaining oxygen from water.
Hibernation: State of rest or inactivity during the cold winter months.
Invertebrate: An animal lacking a spinal column.
Larva: An animal in its early form that does not resemble the parent and must go through metamorphosis, or change, to reach its adult stage.
Vertebrate: An animal having a spinal column.
Not all amphibians follow this pattern of reproduction. Some salamanders live out their entire lives on land, where they give birth to fully formed live young. Others lay their eggs in moist places on the forest floor, where they hatch as tiny versions of the adults. Some newts retain their external gills throughout their lives. The red-spotted newt of eastern North American spends its juvenile stage on land as the red eft, returning to water to develop and live as an adult.
Anurans. Frogs and toads make up the order Anura, the largest group of living amphibians, comprising about 3,000 species. Anurans lack tails and have long hind legs that are well adapted for jumping and swimming. Most anurans live in areas where there is freshwater, although some are well adapted to drier habitats. Some common anurans of North America
include the bullfrog, spring peeper, American toad, and spadefoot toad. Frogs and toads differ in that toads have shorter legs and drier skin that appears warty in comparison to the smooth skin of frogs. Frogs range in size, the smallest measuring about 1 inch (2.5 centimeters) and the largest (the West African Goliath frog) measuring more than 1 foot (about 30 centimeters).
Frogs and toads live mainly on a diet of insects and other invertebrates. The largest frogs and toads also eat small mammals, birds, fish, and other amphibians.
Urodeles. The order Urodela contains about 250 species of newts and salamanders. Urodeles range in size from approximately 4 inches (about 10 centimeters) to the largest of all amphibians, the giant salamander of Japan, which grows to more than 5 feet (about 1.5 meters). Urodeles have long tails and small, underdeveloped legs. They are usually found in or near water and often reside in moist soil under rocks or logs. Adults usually spend most of their time on land and have a diet consisting of insects and worms.
Some species of urodeles are aquatic (live in water), including those of the genus Siren. These North American amphibians are shaped like eels, have small forelegs and no hind legs or pelvis. They breathe through external gills as well as lungs and burrow in mud at the bottom of marshes.
Gymnophions. Caecilians of the order Gymnophiona are blind, legless amphibians shaped like worms. They burrow in moist soil in tropical habitats of Africa and South America, feeding on soil invertebrates such as worms. There are at least 160 species of caecilians, ranging in size from 4 inches (about 10 centimeters) to 4.5 feet (about 1 meter) in length, but most are rarely seen despite their size.
In the last half of the twentieth century, scientists noted the alarming decline in the numbers of amphibians and amphibians species around the world. They theorized the decline was due to a number of factors: pollution of freshwater ecosystems, the destruction of amphibian habitats by ever-spreading human populations, and, possibly, increased ultraviolet radiation due to ozone depletion. Amphibians are known as indicator species, or species whose health is an indicator or sign of the health of the ecosystem they inhabit. As their numbers decrease, so do the number of healthy ecosystems around the world, which in turn results in the loss of many other animal and plant species.