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Amphibians

They can be found under the damp logs on forest floors; propelling themselves through the waters of streams, lakes, and ponds; burrowing into the ground; bounding through woodlands with great leaps; or waddling across dry, sandy desert. These smooth-skinned vertebrates (animals with backbones) called amphibians inhabit all parts of the world except Greenland and Antarctica.

A frog called the Spring Peeper
A Spring Peeper.
Photo courtesy of the U.S. Fish and Wildlife Service
There are three main groups of amphibians: frogs and toads, salamanders and newts, and caecilians. As they go through their early growing stages, the three groups look similar; but as adults, they look very different. Frogs and toads have four legs and no tails. Their hind legs are long and strong. They use their well-developed back legs for jumping and swimming. Salamanders and newts have tails their entire lives. They also have four legs as frogs and toads do. However, their short, weak legs are suited to walking rather than jumping. Caecilians, which have no legs, are burrowing animals that look a lot like giant earthworms.

Scientists have identified more than 4,000 kinds, or species, of amphibians. More species are still being discovered, particularly in the rain forests of Africa, South America, and Southeast Asia.

Characteristics of Amphibians
Amphibians are ectothermic, that is, they depend on external heat sources, such as the sun, to raise their body temperature. Most amphibians spend the early part of their lives in water and the adult part on land. During the early, aquatic period, amphibians use gills for respiration. Later on land, most breathe through lungs. Amphibians can also absorb oxygen through their skin, which is generally smooth, moist, and scaleless. In some species of salamanders, the lungs do not function; all respiration is through the skin and the mucous membranes of the mouth.

Senses
Amphibian senses are well developed. Frogs and toads have excellent sight and depend on their eyes to find food. Salamanders have good eyes and also a keen sense of smell. When they hunt for food, they use both sight and smell. For the burrowing life of caecilians, good eyesight is not a necessity. They either have very small eyes or no eyes at all. Caecilians hunt mainly by smell. They have two tentacles, or feelers, near the mouth. As they move through the ground, the tentacles pick up food scents.

Eating Habits
In their aquatic larval stage, young amphibians vary in their diets. Some eat mainly plant material. Others eat only insects and other animals. Still others eat a mixture of plant and animal food. Adult amphibians, however, are almost entirely meat eaters and make insects the bulk of their diet.

Producing Sounds
Most amphibians are able to produce some kind of sound. Frogs and toads use their voices to communicate warnings, defend themselves, and send mating calls. However, except for frogs and toads, amphibians do not have well-developed voices. Caecilians and most salamanders and newts are limited to sounds such as coughs and grunts. The sounds they produce are not used as a form of communication.

Defenses
Many different kinds of mammals, snakes, and birds hunt the amphibian. When threatened, amphibians that closely match the colors in their environment may hide from their enemies by simply staying very still. Some salamanders have tails that break off. The salamander quickly escapes as its attacker chases the twitching tail. Many amphibians have poison-secreting skin glands. The poison irritates the mouth of the attacker, and it does not take much time before the attacker willingly lets go of its prey.

Life Cycles
In most species of amphibians, the young animals, or larvae, that hatch from the eggs resemble fish in certain ways. They must live in water. They have tails that they use to propel themselves. They take in oxygen through gills. Gradually, the larvae undergo changes that turn them into land-dwelling adults. Such a change of form is called a metamorphosis.

As full-grown adults, most amphibians are small creatures. They usually are no longer than 6 inches (15 centimeters) and weigh less than 2 ounces (57 grams). The largest amphibian is the Japanese giant salamander, which grows to 5 1/4 feet (1.6 meters) in length. One species of caecilian also reaches lengths of more than 4 feet (1.2 meters).

Frogs and Toads
Frogs and toads are the most widespread of all the amphibians. They usually live in wet or moist places such as ponds, marshes, or rain forests, but there are some toads that are able to live in dry areas, including deserts. In frogs and toads, fertilization of the eggs is almost always external. The male fertilizes the eggs as they are being released into the water by the female.

The larva that hatches from an egg is called a tadpole or polliwog. It has a tail and gills. The larva grows and develops over a period of time that lasts from two weeks to two years; then, metamorphosis to the land stage begins. The gills are absorbed and the animal develops lungs. Legs develop and the tail disappears. Finally the animal has the characteristics of an adult frog. It climbs out of the water and onto land. It continues to grow until it reaches its full size. When it is mature, it returns to the water to mate and breed.

There are exceptions to this typical pattern. In one species, for example, the male frog carries strings of eggs wrapped around his hind legs until they hatch. In other species, there is no aquatic larval stage; the froglets that hatch from the eggs look like miniature adults.

Salamanders and Newts
Salamanders and newts generally live in lakes and streams and under damp logs on forest floors. In most tailed amphibians, fertilization occurs internally within the female's body. After courtship, the male deposits a packet of sperm, called a spermatophore, on the bottom of the pond. In species that breed on land, the male places the spermatophore on moist ground. The female picks up the spermatophore and stores it in her cloaca, a chamber at the base of the tail that opens to the outside of the body. When the female lays her eggs, they pass through the cloaca, where the sperm fertilize them.

The larvae of salamanders and newts are similar in appearance to the adults. The main difference is that the young animal has feathery gills on the outside of its neck. After two to four months, the larva changes into an adult. It loses the gills and forms air-breathing lungs.

Caecilians
The caecilians are found only in the tropics and subtropics. Most live in underground burrows; however, there are caecilians that spend all their life in water.

Fertilization is internal in caecilians. Most species deposit their eggs in water. Some caecilians lay eggs on land. After depositing her eggs, the female caecilian may coil herself around them to keep the eggs safe from predators until they hatch. The caecilian larvae have tails and live in water. Like frogs and toads, they gradually lose their tails and turn into adults.
   
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