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Frog Finds
From Grolier's The New Book of Knowledge
The hyla microps, which Eve studied in the rain forests of Brazil.

Photo: Magno Segalia

On a warm March evening, shortly after the first signs of spring appear, the spring peeper begins his search for a mate. He journeys to a familiar breeding site. There, he joins hundreds of other males as they sing their mating call. The clear, full chorus of these tiny frogs, which are about an inch long, can be heard almost a mile away!

The spring peeper is just one of the more than 3,700 species, or kinds, of frogs and toads that scientists have identified. Frogs and toads have small, tailless bodies with bulging eyes and well-developed hind legs that are used for leaping or hopping. They belong to a group of vertebrates (animals with backbones) called amphibians. Like other amphibians, frogs and toads generally spend part of their life in water and part on land.

Frogs and toads are found on every continent except Antarctica, but they are most common in the tropics. They occupy a variety of habitats. Most species live in ponds, marshes, rain forests, and other wet places. But some toads live in dry places, such as deserts.

The Characteristics of Frogs and Toads

Frogs and toads do not make their own body heat. Their body temperature depends on the temperature of their environment—that is, their inside temperature changes when the outside temperature changes. Animals that rely on external heat sources, such as the sun, to raise or lower their body temperature are called ectothermic.

To survive, ectothermic animals must avoid temperature extremes. Frogs and toads that live in places with freezing temperatures hibernate to escape the cold. They stay underground, beneath piles of leaves, or they burrow into the muddy bottoms of ponds or streams where the temperature stays above freezing. In places with high temperatures, frogs and toads escape the heat in a similar way. Toads that live in deserts escape the burning rays of the sun by staying underground during the day. When the night comes and the temperature goes down, they come to the surface.

Body Structure. The basic body structure of frogs and toads is the same. They have short, round neckless bodies with large flat heads and four legs. In general, frogs have longer, stronger legs than toads.

Eating Habits. Frogs and toads are not fussy eaters. As adults, they are carnivores, or meat eaters. Their diet consists mainly of insects. They also eat spiders, worms, and other frogs.

They use their excellent eyesight to find prey. When a frog or toad sees a fly or other prey, it whips out its long tongue. The tongue is elastic and can stretch quite a distance. It catches the prey on its tongue's sticky tip. Then the tongue and prey are quickly flicked into its mouth.

Some aquatic frogs do not have tongues. They use their fingers to find small animals in the mud. They scoop up the prey and carry it to the mouth.

Producing Sounds. During the breeding season, frogs and toads are very noisy. Thousands of males form choruses as they call. They call to attract females. Each species has its own mating call: The pig frog is named for its piglike grunt, the distant call of the eastern barking frog sounds like a barking dog, the bullfrog calls "jug-o-rum," and the Mexican burrowing toad calls "whooooa."

The sound made by a frog or toad is produced when air is forced over its vocal cords. The vocal cords are thin bands of tissue located in the larynx, or voice box, which lies between the mouth and lungs.

In addition to vocal cords, males in some species have special throat pouches called vocal sacs. When the animal gulps air, the sacs swell up like balloons. These air-filled vocal sacs help produce the sounds that make a male's call distinctive.

Mating calls are not the only sounds made by frogs and toads. They also use their voices to claim territory, to warn away rivals, and to frighten off predators.

Defenses. One of the best defenses of frogs and toads is the color of their skin. It provides them with natural camouflage: Many frogs and toads are green or brown, so they blend in with the color of the plants or rocks on which they sit. This makes it difficult for enemies and prey to see them. Some frogs and toads can change color. When the European green toad sits among grasses, it is tan with many green spots. When it moved onto soil, it turns a grayish brown.

Nearly all frogs and toads have poison-secreting glands in their skin. An animal that tries to eat a frog or toad may spit it out because the poison irritates and burns the inside of its mouth.

The Life Cycle of Frogs and Toads

Most species of frogs and toads mate in water. The male journeys to the mating place first. He calls to the female to join him. Once the female arrives, the pair come together in a mating embrace. As they cling together, the female releases her eggs into the water. Then the male releases sperm, which fertilize the masses of eggs. Most frog and toad parents abandon the fertilized eggs, leaving them in constant danger from predators. Out of the thousands of eggs that are laid and the young tadpoles that hatch from them, only a few will escape predators and develop into adults.

As a newly hatched tadpole grows, it undergoes a metamorphosis. That is, it changes form in stages—from a fishlike creature to a frog. Fully grown frogs and toads are usually not more than 6 inches (15 centimeters) in length or 2 ounces (57 grams) in weight.

Frogs and Toads and Their Environment

Frogs and toads play important roles in the environment. They are food for many kinds of animals. Fish feed on tadpoles. Birds, mammals, and snakes eat frogs and toads. People eat frogs, too—the legs of large frogs such as the bullfrog are a delicacy in many countries.

Frogs and toads provide other services, such as controlling insect pests, that make them valuable to people. They eat enormous numbers of insects, including many insects that carry disease and destroy crops. They are also used in scientific research and teaching. Many drugs are first tested on frogs.

In many places, frogs and toads are disappearing because of human activity. Automobiles kill thousands of frogs that must cross roads to reach their breeding places. Their habitats disappear as bodies of water are drained or polluted and as natural areas are destroyed to provide land for development.

In the 1990's, people began noticing unusual deformities in many frog populations. Some scientists think that parasitic worms or pesticides are to blame. Others suggest that holes in the atmosphere's ozone layer are letting in more of the sun's ultraviolet radiation, which may be harming amphibians.

Frogs and toads are very sensitive to changes in their environment. If they are in trouble, so are the animals around them—including humans. Perhaps the most important role of these amphibians will be their early warning that our environment, like theirs, is now in trouble. Let us hope their voices, though small, will continue to be heard.

Jenny Tesar
Author, Introduction to Animals

Reviewed by John L. Behler
Curator, Department of Herpetology, The Bronx Zoo

Copyright © 2002 Grolier Incorporated. All Rights Reserved.