Frogs
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.