Like higher animals, frogs have such internal organs as a heart, liver, lungs, and kidneys. However, some of the internal organs differ from those of higher animals. For example, a frog's heart has three chambers instead of four. And although adult frogs breathe by means of lungs, they also breathe through their skin.

Legs. A few burrowing species have short hind legs and cannot hop. But all other frogs have long, powerful hind legs, which they use for jumping. Many frogs can leap 20 times their body length on a level surface. Frogs also use their large hind legs for swimming. Most water-dwelling species have webbed toes on their hind feet. The smaller front legs, or arms, prop a frog up when it sits. The front legs also help break the animal's fall when it jumps. Frogs that live in trees have tiny, sticky pads on the ends of their fingers and toes. The pads help the animal cling to the tree trunk as it climbs.

Skin. Most frogs have thin, moist skin. Many species have poison glands in their skin. The poison oozes onto the skin and helps protect the frog. If an enemy grabs a frog, the poison irritates the enemy's mouth and causes the animal to release the frog. Frogs have no hair, though the males of one African species, the so-called hairy frog, look hairy during the mating season. At that time, tiny, blood-rich growths called papillae, which resemble hair, grow from the sides of the frog's body. These structures provide males with extra oxygen during a period when they are very active.

Some species of frogs change their skin color with changes in the humidity, light, and temperature. Frogs shed the outer layer of their skin many times a year. Using their forelegs, they pull the old skin off over their head. They then usually eat the old skin.

Senses. Frogs have fairly good eyesight, which helps them in capturing food and avoiding enemies. A frog's eyes bulge out, enabling the animal to see in almost all directions. Frogs can close their eyes by pulling the eyeballs deeper into their sockets. This action closes the upper and lower eyelids. Most species also have a thin, partly clear inner eyelid attached to the bottom lid. This inner eyelid, called the nictitating membrane, can be moved upward when a frog's eyes are open. It protects the eyes without completely cutting off vision.

Most frogs have a disk of skin behind each eye. Each disk is called a tympanum, or eardrum. Sound waves cause the eardrums to vibrate. The vibrations travel to the inner ear, which is connected by nerves to the hearing centers of the brain.

Most frogs have a delicate sense of touch. It is particularly well developed in species that live in water. The tongue and mouth have many taste buds, and frogs often spit out bad-tasting food. The sense of smell varies among species. Frogs that hunt mostly at night or that live underground have the best sense of smell.

Voice. Male frogs of most species have a voice, which they use mainly to call females during the mating season. In some species, the females also have a voice. But the female's voice is not nearly so loud as the male's.

A frog produces sound by means of its vocal cords. The vocal cords consist of thin bands of tissue in the larynx (voice box), which lies between the mouth and lungs. When a frog forces air from its lungs, the vocal cords vibrate and give off sound.

Among many species, the males have a vocal sac, which swells to great size while a call is being made. Species that have a vocal sac produce a much louder call than do similar species that have no sac. Some species have a vocal sac on each side of the head. Others have a single sac in the throat region.

The life of a frog

Frogs, like all other amphibians, are cold-blooded--that is, their body temperature tends to be the same as the temperature of the surrounding air or water. Frogs that live in regions with cold winters hibernate. Some species hibernate in burrows. Others spend the winter buried in mud at the bottom of a pond or stream, breathing through their skin. During hibernation, a frog lives off materials stored in its body tissues.

Mating. Most frogs that live in tropical and semitropical regions breed during the rainy season. In other regions, most species of frogs breed in spring or in early summer.

The majority of frogs, including most species that live on land, mate in water. The male frogs usually enter the water first. They then call to attract mates. Their call also helps direct other males to a pool suitable for mating. Each species has its own mating call. Naturalists can identify many kinds of frogs more easily by their call than by their appearance. Female frogs respond only to the call made by males of their own species. In certain species, individual differences in the mating call may determine which male the female chooses to mate with. Males of some species also have a territorial call. This call warns other males of the same species that a certain area is occupied and that intruders are not welcome.

After a female frog enters the water, a male grasps her and clings to her back. In this position, the male fertilizes the eggs as they leave the female's body. The eggs hatch within 3 to 25 days, depending on the species and the water temperature. Higher water temperatures speed up development, and lower temperatures slow it down. Among most species, a tiny, tailed animal known as a tadpole or polliwog hatches from the egg.

Kinds of frogs

Frogs and toads make up the order Anura, or Salientia, one of the three main groups of amphibians. Most zoologists divide this order into at least 19 families of living frogs and toads.

One family of anurans consists of true frogs. True toads make up another family. Most true toads have a broader, flatter body and darker, drier skin than do most true frogs. True toads are commonly covered with warts, but true frogs have smooth skin. Unlike most true frogs, the majority of true toads live on land. The adults go to water only to breed. For more information on true toads, see TOAD.

Of the other families in the order Anura, some closely resemble true frogs, and others closely resemble true toads. Still others have features of both true frogs and true toads. Certain anuran families other than the true toads also have the word toad as part of their common name.

Some of the most common frogs in the United States and Canada belong to the true frog and tree frog families.

True Frogs live on every continent except Antarctica. They are most common in Africa. The majority of true frogs live in or near water. They have long hind legs, smooth skin, a narrow waist, and webbed hind feet.

More than 20 kinds of true frogs live in the United States. Many of these frogs also live in Canada. A group of related species known as leopard frogs are the most widespread. Leopard frogs range from the Atlantic coast to eastern California and from northern Canada to the Mexican border. The bullfrog, which may grow up to 8 inches (20 centimeters) long, ranks as the largest American and Canadian frog. Other common true frogs of the United States and Canada include the green frog, the pickerel frog, and the wood frog. Unlike most other true frogs, the wood frog spends much of its time away from water. It lives in damp wooded areas of Alaska, Canada, and the Midwestern and Eastern United States.

Tree Frogs, like true frogs, live on all continents except Antarctica. Most tree frogs measure less than 2 inches (5 centimeters) long and dwell in trees.

About 25 species of tree frogs live in the United States. Some of these species are also found in Canada. Common species in the Eastern United States include the green tree frog, the gray tree frog, and the spring peeper. Western tree frogs include the California tree frog, the canyon tree frog, and the Pacific tree frog. Some North American tree frogs, called chorus frogs and cricket frogs, live mainly on the ground.

Other Frogs of the United States include leptodactylid frogs, narrow-mouthed toads, spadefoot toads, and tailed frogs.

Leptodactylid Frogs make up a large family of frogs that live mainly in Australia and South America. Those found in the United States include the barking frog, the cliff frog, and the white-lipped frog. The barking frog and the cliff frog live on rocky cliffs in Texas. These frogs lay their eggs under rocks. Tiny frogs hatch from the eggs, without going through the tadpole stage. The white-lipped frog lives in the southern Rio Grande Valley area of Texas. The female white-lipped frog lays her eggs in a hole near water. She then beats the egg jelly into a foam. The tadpoles live in the foam nest until rain washes them into the nearby water.

Narrow-Mouthed Toads live throughout most tropical and subtropical regions. As their name suggests, these frogs have an extremely narrow mouth. The eastern narrow-mouthed toad, the Great Plains narrow-mouthed toad, and the sheep frog are the only members of this family that live in the United States. All three species live in burrows and eat ants and termites.

Spadefoot Toads live in Asia, Europe, North America, and northwestern Africa. These frogs are called spadefoots because most of them have a sharp-edged spadelike growth on each hind foot. They use this growth as a digging tool.

Spadefoot toads live throughout much of the United States. They dwell underground and are usually seen only after a rain. Several species live in dry regions of the Great Plains and the Southwest. These spadefoots may remain in their burrows for weeks at a time to stay moist. They breed following heavy rains, often laying their eggs in temporary ponds. The tadpoles develop rapidly. If enough food is available, tiny adults may emerge in only 12 days.

Tailed Frogs live in swift mountain streams of the northwestern United States and southwestern Canada. The moving water makes external fertilization of the eggs difficult. Instead, the male uses a taillike structure to fertilize the eggs while they are inside the female. Tadpoles of tailed frogs have a large sucker that enables them to hold on to rocks even in the strongest current.

Frogs and human beings

Frogs benefit us in many ways. They eat large numbers of insects, which might otherwise become serious pests. Frogs also provide us with food. The meaty hind legs of larger frogs are considered a delicacy in many countries. In the United States, people mainly eat the legs of bullfrogs, green frogs, and leopard frogs. Frogs also are used widely in the laboratory. Medical researchers use frogs to test new drugs, and students dissect frogs to learn about anatomy.

Human beings are, in fact, the frog's worst enemy. People obtain most of the frogs used for food and in the laboratory from the wild. Furthermore, people destroy the homes and breeding places of frogs by replacing natural areas with cities and farms. They also pollute and so poison the waters in which frogs dwell.

Scientific classification. True frogs make up the family Ranidae. All North American true frogs are in the genus Rana. Tree frogs make up the family Hylidae. Leptodactylid frogs make up the family Leptodactylidae; narrow-mouthed toads, the family Microhylidae; and spadefoot toads, the family Pelobatidae. The tailed frog belongs to the family Leiopelmatidae.