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 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 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.

Bibliography

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.