A Frog is a small, tail less animal that has bulging eyes. Almost all frogs have long back legs. The strong hind legs make the frog able to leap farther than the length of its body. Frogs live on every continent except Antarctica, but tropical regions have the greatest number of species. Frogs are classified as amphibians. Most amphibians, including most frogs, spend part of their life as a water animal and part as a land animal.
Frogs are related to toads, but are different from them in a few ways. The giant frog of west-central Africa ranks as the largest frog. It measures nearly a foot (30 centimeters) long. The smallest species grow only 1/2 inch (1.3 centimeters) long. Frogs also differ in color. Most kinds are green or brown, but some have colorful markings.
Although different species may vary in size or color, almost all frogs have the same basic body structure. They have large hind legs, short front legs, and a flat head and body with no neck. Adult frogs have no tail, though one North American species has a short, tail like structure. Most frogs have a sticky tongue attached to the front part of the mouth. They can rapidly flip out the tongue to capture prey.
Frogs have such internal organs as a heart, liver, lungs, and kidneys. Some of the internal organs differ from those of higher animals. 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.
The eggs of different species vary in size, color, and shape. A jelly like substance covers frog eggs, providing a protective coating. This jelly also differs from species to species. Some species of frogs lay several thousand eggs at a time.
But only a few of these eggs develop into adult frogs. Ducks, fish, insects, and other water creatures eat many of the eggs. Even if the eggs hatch, the tadpoles also face the danger of being eaten by larger water animals The pond or stream in which the eggs were laid sometimes dries up. As a result, the tadpoles die. Certain tropical frogs lay their eggs in rain water that collects among the leaves of plants or in holes in trees. Other tropical species attach their eggs to the underside of leaves that grow over water.
When the eggs hatch, the tadpoles fall into the water. Among some species, one of the parents carries the eggs until they hatch. For example, the female of certain South American tree frogs carries the eggs on her back. Among another species of frog, the midwife toad, the male carries the eggs wound around his hind legs. Males of another species, Darwin's frog, carry the eggs in their vocal pouch. Some tropical frogs lay their eggs on land.
They lay them under logs or dead leaves. These frogs have no tadpole stage. A young frog hatches from the egg and begins life as a land animal. Tadpoles are not completely developed when they hatch. At first, the tadpole clings to some support in the water, using its mouth or a tiny sucker. A tadpole has no neck, and so its head and body look like one round form.
The animal has a long tail and resembles a little fish. It breathes by means of gills, which are hidden by a covering of skin. A tadpole's form changes as the animal grows. The tail becomes larger and makes it possible for the animal to swim about to obtain food. Tadpoles eat plants and decaying animal matter. Some tadpoles eat frog eggs and other tadpoles.
In time, the tadpole begins to grow legs. The hind legs appear first. Then the lungs begin to develop and the front legs appear. The digestive system changes, enabling the frog that develops to eat live animals. Just before its change into a frog, the tadpole loses its gills. Finally, a tiny frog, still bearing a stump of a tail, comes up from the water.
Eventually, the animal absorbs its tail and assumes its adult form. After a frog becomes an adult, it may take a few months to a few years before the animal is mature enough to breed. The green frog and the pickerel frog mature in about three years. In captivity, a bullfrog may live more than 15 years. But few species of frogs live longer than 6 to 8 years in the wild.
Many are eaten by such enemies as bats, herons, raccoons, snakes, turtles, and fish. Adult frogs eat mainly insects and other small animals, including earthworms, minnows, and spiders. Most frogs use their sticky tongue to capture prey. The tongue is flipped out of the mouth in response to movement by the prey. Most frogs have teeth only on their upper jaw. Toads lack teeth altogether.
As a result, frogs and toads swallow their prey in one piece. To aid in the swallowing process, the frog's eyes sink through openings in the skull and force the food down the throat. 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 spade like 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 tail like 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.
Wright, Albert H. and A.A. Handbook of Frogs and Toads of the United States and Canada. 3rd ed. 1949.
Reprint. Cornell Univ. Pr., 1995.