Frogs: A Comprehensive Guide to Amphibians

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Introduction to Frog

Frog, a small tailless amphibian. Scientifically, there is no distinction between frogs and toads. Both belong to the same order of the class Amphibia, and are collectively called anurans. Popularly, anurans that live in or near freshwater are usually called frogs. They vary in size from certain South American frogs less than one-half inch (13 mm) long to the 10-inch (25-cm) goliath frog of Africa.

Most frogs are green or greenish brown, with various markings in darker or lighter color; some frogs, however, are brightly colored. The frogs of the United States and Canada are smooth-skinned, but in other parts of the world there are frogs with rough or irregular skins. Many kinds of frogs shed their entire skin periodically.

Frogs have webbed hind feet and most are fast swimmers.

Are All Frogs Green?

Not all frogs are green. Frogs can be green, gray, red, blue, yellow, purple, orange, or combinations of two or more colors. Whether their skin is plain or eye-catching, it serves as an adaptation: it helps them stay safe.

Some frogs use their skin as camouflage, to help them blend into their environment. Certain frogs even change their skin color with changes in humidity, light, and temperature.

Highly toxic frogs display bright colors to warn predators (PRED uh tuhrz) not to eat them. And some nontoxic frogs display bright colors so predators will think they’re poisonous, too.

Frogs shed the outer layer of their skin many times a year. So do toads. They use their forelegs to pull the old skin off over their heads. Then they eat it.

A frog can breathe not only with its mouth and lungs but also through its skin. In order to breathe through the skin, it must keep its skin moist. Some frogs stay near water; others have developed some adaptation to prevent drying out, such as burrowing into the soil or hiding under rocks. A frog's skin is permeable to water as well as air, and for this reason, a frog can easily absorb harmful chemicals from its environment. As wetlands are dried up by developers, populations of frogs are threatened with extinction.

What’s Under All That Thin, Wet Skin?

A frog is a vertebrate, or an animal with a backbone. Like other vertebrates, a frog also has organs, such as a heart, a liver, kidneys, and lungs.

But a frog is different from other vertebrates. A frog’s heart has three chambers, not four. Frogs can breathe through their skin as well as their lungs.

All frogs have the same basic body structure. They have short front legs and flat heads and bodies. After they grow out of the tadpole stage, frogs don’t have tails.

Most frogs have large back legs, which help them hop from place to place. But a few frogs that dig burrows have short hind legs. They can’t hop at all.

All frogs have short front legs and large, strong hind legs for jumping. The hind feet are webbed, and most frogs are fast swimmers. The flying frog of Asia has both front and hind feet webbed, and uses them to glid from trees to the ground. Tree-climbing frogs have suction pads on their toes. Frogs have well-developed senses of sight and hearing. The male of each species has a distinctive call, used in finding a mate. Frogs of many species secrete poisonous substances as a defense against predators.

Are All Frog Feet the Same?

Different kinds of frogs have different kinds of feet. Frogs that live in or near water have webbed toes on their hind feet. The webbing helps them swim quickly through the water. Flying frogs, which glide from tree to tree, have extra webbing on their front and hind feet. This extra webbing helps the frogs glide through the air.

Some tree frogs have no webbing at all on their feet. Instead, these frogs have sticky pads on the ends of their toes. The pads help them cling to the surface of tree trunks, twigs, and leaves as they climb.How Far Can Frogs Jump?Some frogs can make impressive jumps. Many frogs can leap 20 times their own body length. Some tree frogs can jump over 7 feet (2 meters). That’s nearly 50 times their body length. That is like a 6-foot (1.8-meter) human jumping 300 feet (90 meters).

Not all frogs are great jumpers, however. Some frogs that dig burrows, for example, have short hind legs. Many of them hop weakly, if at all.

Frogs are found in the temperate and tropical regions of all continents. They cannot live in extremely cold climates or at high altitudes.

Where in the World Do Frogs and Other Amphibians Live?

Frogs have lived on Earth for about 180 million years, and they’ve developed ways to live on every continent of the world except Antarctica. Today, most frogs live in tropical regions.

Different kinds of frogs live in different habitats. Members of the common group of frogs, known as “true frogs,” usually live in or near water. They have long hind legs, smooth skin, and webbed hind feet.

Tiny tree frogs dwell in—you guessed it—trees. Other frogs live on rocky cliffs, or in swift mountain streams. Some kinds of frogs live in burrows and eat ants and termites.

Many kinds of frogs and other amphibians live their whole lives in or near water. But some leave the water when they become adults and only return to mate and lay eggs or bear their young. A few types of frogs never enter the water.

Why Are Frogs So Cold?

Like all amphibians, frogs are ectotherms (EHK tuh thuhrmz). They can’t warm themselves up or cool themselves off. They have to rely on their habitat to do that for them.

People say frogs are cold-blooded animals, but their blood isn’t really cold. Their body temperature tends to be the same as the temperature of the surrounding air or water.

When the weather gets too hot and dry, frogs often estivate (EHS tih vayt), or sleep underground away from the heat. When they sense moisture in the soil, they awaken and come to the surface.

Frogs that live in areas that have winter hibernate (HI buhr nayt) to escape the cold. They may burrow under leaves or mud at the bottom of a pond or stream. Or they may dig down as much as a foot underground, often under rocks or rotten logs.

The hind legs of any species of frogs are prized as food. Indians in Colombia obtain poison from the skin of certain species of poison-dart, or poison-arrow, frogs and apply it to blowgun darts they use in hunting.

Are Some Frogs Poisonous?

Most frogs are harmless, but some frogs, as well as some toads, have glands that ooze poison onto their skin. The poison dart frogs of Central and South America make a poison on their skin strong enough to kill almost any predator. Indians in this region have painted this poison on arrows to make them deadly.

Most of the other kinds of poisonous frogs emit only a mild poison. Some of these frogs simply taste so bad because of the poison that no one wants to eat them. But others are deadly to small predators. Often, the brighter the frog is, the more deadly the poison it emits.

Mildly poisonous frogs are usually harmless to humans, but even mild poisons can irritate your eyes. Be sure to wash your hands after handling a frog. Also, keep frogs and toads away from pets. Poison that may only irritate a human could have a worse effect on a smaller animal.

Habits of Frogs

Life Cycle

With few exceptions, frogs lay their eggs in water and spend the early part of their lives in water. In temperate zones, most frogs hibernate in mud. In spring or summer, the female lays thousands of tiny eggs in large masses. The eggs, fertilized by the male as they are discharged by the female, are held together by a jellylike substance that swells in water to form a protective coating.

Frogs have distinctive calls used to find mates.How Do Frogs Find Mates?

During breeding season, when they want to find a mate, frogs often return to the place where they hatched. Usually, the males arrive first. Each male croaks loudly to establish its own territory. In many species, when the females arrive, the males puff up vocal sacs in their throats and make a special mating call. They wait for females to respond.After a female chooses a male, he climbs onto her back and holds onto her in the water. The female lays eggs and the male produces a milky substance that is poured over the eggs to fertilize (FURH tuhl lyz) them.

After a period of from several days to several weeks (varying with species, water temperature, and amount of sunshine) tadpoles hatch out. The tadpole, or polliwog, is a frog in the larval stage. It has a tail and breathes with gills. At first the tadpole is legless, but as it develops it acquires first hind legs and then front legs. The tail is gradually absorbed, lungs develop, and the animal is metamorphosed (changed) into an adult frog. An adult attains full growth in about three years and may have a life span of 10 years or more.

Is That Thing a Frog?

A tadpole doesn’t look much like a frog—but when it grows up, that’s exactly what it will be. Most types of frogs spend the first stage of life as tadpoles, which is simply another name for frog larvae.

Tadpoles hatch out of eggs that are covered by a thick, jellylike coat that keeps them moist. At first, tadpoles look a lot like fish. They have gills, lidless eyes, and finlike tails for swimming.

Some kinds of tadpoles go through metamorphosis in about 10 days. Others take as long as two years to change form completely.

During metamorphosis, a tadpole first grows hind legs. Then forelegs appear from under the gills. The mouth widens and a tongue develops. Then the lungs develop and the gills disappear. The skin gets thicker and tougher, nostrils form, and the body absorbs the tail: it disappears! The tadpole has become a frog.

Some frogs do not lay eggs in water. A frog found in the Solomon Islands lays large, tough-shelled eggs from which fully formed froglets are hatched. In some species of tree frogs, the female lays the eggs on leaves or carries them in a pouch on her back where they eventually hatch.

Feeding

Tadpoles feed mainly on algae. After metamorphosis, the digestive system changes, and the frog eats flies, worms, slugs, spiders, caterpillars, and small beetles. The bullfrog is large enough to swallow crayfish and small birds. Some kinds of tropical frogs eat rats, bats, and snakes.

Frogs have strong hind legs for jumping and swimming.

Most frogs have long, sticky tongues that can be rapidly extended to catch prey, which is swallowed whole. The teeth, located in the upper jaw and roof of the mouth, grasp the prey until it is swallowed.

What Do Frogs Eat?

Frogs in the wild will eat almost anything they can catch—but only if it’s still alive. Most frogs eat insects, snails, and tadpoles. Large frogs will also eat small mammals, birds, fish, and reptiles.Most frogs have a keen ability to spot movement. Once a frog recognizes a moving object as prey, it tries to catch it. Some frogs can flick their tongues out in a fraction of a second to grab their food. Other frogs use their jaws to trap their prey.

In the water, a frog may simply open its mouth and gulp its prey. Like most other amphibians, frogs have teeth. But they only use them to grip food, not to chew it. If a frog catches something that tastes bad, it spits it out. But if the frog likes its catch, it swallows it whole.

Frogs of the U.S. and Canada

True frogs—the ones discussed in this article—belong to the family Ranidae. There are about 20 species in the United States and Canada, including the following:

Bullfrog

With a body up to 8 inches (20 cm) long, this is the largest frog in North America and next to the largest in the world. The skin is green, brown, or black. The bullfrog is found in Canada, the eastern United States, and northern Mexico. Its deep, hoarse call is louder than that of any other species of frog. Bullfrogs are raised commercially for their legs, which are eaten fried or baked.

The bullfrog is Rana catesbeiana.

The bullfrog is up to eight inches long and has a deep, hoarse call.Green Frog

It is 2 to 4 inches (5 to 10 cm) long. The back is greenish-brown or bronze with numerous dark brown or gray spots; the belly is white. Ridges extend along the sides of the body. The green frog ranges from the Great Plains to the Atlantic, and from southern Canada to Mexico. The male's call is a low-pitched “Ctung! Ctung!”

The green frog is R. clamitans.

Northern Leopard Frog

It is 2 to 3 1/2 inches (5 to 9 cm) long and is brown or green. Between the ridges that extend along the sides are two or three rows of dark spots. There is a light stripe on the upper jaw. The northern leopard frog is widely distributed over the northern United States and southern Canada. The male's call is a low snoring sound.

The northern leopard frog is R. pipiens.

Pickerel Frog

It is 1 3/4 to 3 1/2 inches (4.5 to 9 cm) long. The skin is tan with two parallel rows of squarish spots down the back. The belly and hind legs are bright yellow or orange. The skin secretes a substance that is poisonous to predators. The pickerel frog inhabits swamps and meadows in the eastern United States. The male's call is a low croak.

The pickerel frog is R. palustris.

River Frog

It is 3 to 5 1/2 inches (7.5 to 14 cm) long. The skin is olive green or black and has a rough texture. The male has a yellow throat. The skin secretes a substance that is poisonous to predators. The river frog inhabits swamps from South Carolina to central Florida. The male makes a grunting or snoring sound.

The river frog is R. heckscheri.

Wood Frog

With a body only 1 1/2 to 2 3/4 inches (4 to 7 cm) long, it is one of the smallest frogs. The skin is tannish-pink with dark patches around the eyes and at the base of the forelegs. The wood frog is one of the first amphibians to appear in the spring. It inhabits moist woodlands in the northeastern United States and throughout Canada. The male makes a raspy quacking sound.

The wood frog is R. sylvatica.

How Do Frogs Protect Themselves?

In addition to coloration that camouflages them or warns that they’re poisonous, frogs have good eyesight. They also have a delicate sense of touch to help them keep track of their surroundings.

If a predator spots a frog, the frog might play dead because most predators prefer live food. If it’s too late to hide or play dead, a frog may try to scare its enemy and get away. For example, some tree frogs flash brilliant colors to startle their enemies. Budgett’s frog of Argentina and Paraguay bellows, screams, and grunts to scare foes away.

Frogs that live on the ground or in water have powerful legs that can help them hop or swim away from predators. Tree frogs can escape danger by leaping from branch to branch. Flying frogs don’t actually fly, but they can jump right out of trees and glide to safer places.

How Do Frogs Help Humans?

Frogs are helpful to humans in a number of ways. Frogs eat many insects that are pests, such as mosquitoes and flies. They also eat insects that harm crops.

In addition, because their bodies are so similar to the bodies of other vertebrates, students and scientists often study frogs. Recently, scientists learned that the chemicals some frogs secrete can be used in medicine to fight skin and eye infections.

Frogs are helpful in another way, too. Because they absorb water through their skin, they are often the first creatures to react to pollutants in the water. Frogs that get diseases or can no longer reproduce may give a warning to people about the harmful effects of pollutants in the environment.

Do Frogs Make Good Pets?

Frogs can make good pets. They are colorful, and they are fun to watch as they grow.

But before you get a pet frog, find information at your library or talk to experts at a local pet store. Taking care of frogs can be a lot of work. Some frogs eat only live insects. And frogs can live a long time, so be prepared to care for a pet frog for many years. You should never release a store-bought frog into the wild. It may not be a native to your area, and so it could cause harm to the habitat.

A pet frog can be kept in a mesh- or screen-covered terrarium (tuh RAIR ee uhm), which is a transparent container made of glass or plastic. If your pet frog is a type that prefers a dry environment, small pebbles should be placed at the bottom of the container for drainage. Then potting soil and plants can be added. The completed terrarium should be placed in an area that is well lighted but out of direct sunlight.

In addition to frogs, terrariums can hold other amphibian pets, such as toads and salamanders. Pet terrariums need to be kept clean to prevent illness.