Frog is a small, tailless animal with hairless skin and bulging eyes. Almost all frogs also have long, strong back legs. Such legs enable a frog to leap distances far greater than the length of its body. Frogs live on every continent except Antarctica, but tropical regions have the greatest number of species (kinds). Frogs are classified as amphibians. Most amphibians, including many frogs, spend a part of their lives, known as the larval stage, living in the water and breathing through gills. After undergoing a transformation called metamorphosis, they spend the rest of their lives breathing air. In frogs, the larval stage is called a tadpole or sometimes a polliwog. Frogs and toads are not really different animals. Rather, toad is a familiar term that refers to a specific type of frog.
The first frogs appeared on Earth during the Jurassic Period, which lasted from about 200 million years ago to about 145 million years ago. Thousands of species of frogs have evolved (developed over time) from these early ancestors. Some species spend their entire lives in or near water. Others live mainly on land and come to the water only to mate. Still other species never enter the water, not even to mate. These frogs lack the tadpole larval stage. Many kinds of frogs are climbers that dwell in trees. Others are burrowers that live underground.
Throughout history, frogs have inspired many superstitions. For example, one old myth says that frogs fall from the sky during a rain. Actually, many species that live underground leave their burrows during or after a rain at the start of the mating season. Because people seldom see these frogs the rest of the year, they may imagine that the animals fell from the sky with the rain. Other species, typically referred to as toads, have bumps called tubercles on the skin that resemble warts . Some people believe that toads cause warts on people that handle them. Neither of these superstitions is true.
Frog croaking
The body of a frog
The giant, or Goliath, frog of west-central Africa ranks as the largest frog. It can reach more than 1 foot (30 centimeters) in length. The smallest species reaches less than 1/2 inch (1.3 centimeters) long. Frogs differ greatly in color. While many are some combination of green and brown, some have vivid markings of blue, red, yellow, or other striking colors.
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, a flat head, and no neck. Adult frogs have no tail, though one North American species has a short taillike structure. Most frogs have a sticky tongue attached to the front part of the mouth. They can rapidly flip out their tongue to capture prey.
Like other vertebrates (animals with backbones), frogs have such familiar internal organs as a heart, liver, stomach, intestines, brain, and kidneys. At least one species of frog is known to lack lungs. Frogs typically respire (breathe) through the lungs, the surface of the skin, or the lining of the mouth.
Legs.
A few burrowing species have short hind legs and cannot leap. But all other frogs have long, powerful hind legs, which they use for jumping. Many frogs can leap 20 times their body length on a level surface. Frogs may also use their large hind legs for swimming. Most species that spend time in the water have webbed toes on their hind feet. Some fully aquatic (water-dwelling) frogs are so specialized that they cannot move well on land.
A frog’s smaller front legs, or arms, prop up the frog when it sits. The front legs also help to break the animal’s fall when it jumps. In addition, they sometimes are used to help push food into the mouth.
Frogs that live in trees or among rocks have enlarged sticky pads on the ends of their fingers and toes. The pads help the animal to cling to surfaces as it climbs.
Skin.
Frogs have thin, moist skin that is essential in respiration (breathing). Some species of frogs change their skin color with changes in humidity, light, and temperature. Frogs molt (shed the outer layer of the skin) many times a year. Using their forelegs, they pull the old skin off over their head. They then usually eat the old skin.
Many species of frog have poison glands in their skin. The poison oozes onto the skin and helps protect the frog. If a predator (hunting animal) grabs a frog, the poison irritates the predator’s mouth and causes the animal to release the frog.
Frogs have no hair, though the males of one African species, the so-called hairy frog, look hairy during the mating season. At that time, tiny, blood-rich growths called papillae, which resemble hair, grow from the sides of the frog’s body. These structures provide males with extra oxygen during a period when they are active.
Senses.
Frogs have keen eyesight, which helps them in capturing food and avoiding predators. Frogs are especially good at seeing in extremely dim light. A frog’s eyes bulge out of the top of the head, enabling the animal to see in almost all directions. Frogs can close their eyes by pulling the eyeballs deeper into their sockets. This action closes the upper and lower eyelids and also causes the eyes to bulge into the mouth cavity. Most species also have a thin, partly clear inner eyelid attached to the bottom lid. This inner eyelid, called the nictitating membrane, can be moved upward when a frog’s eyes are open. It protects the eyes without completely blocking vision.
Most frogs have a disk of skin behind each eye called a tympanum, or eardrum. Sound waves cause the tympanum to vibrate. The vibrations travel to the inner ear, which is connected by nerves to the hearing centers of the brain.
A frog’s tongue and mouth have many taste buds, and frogs often spit out bad-tasting food. Frogs appear to have a keen sense of smell, but it is not known how individual species use this sense in their respective environments. The sense of touch has been poorly studied in frogs.
Voice.
Male frogs of most species have a voice, which they use mainly to call females during the mating season. Some species call from underwater, and some species lack a voice altogether. In some species, the females also have a voice. But the female’s voice is not nearly as loud as that of the male. The female’s voice is usually used to discourage unwanted males from mating. A few species, such as the smoky jungle frog of South America, will scream loudly when grasped by a predator. The scream may startle the predator into releasing the frog.
Loading the player...Bullfrog
A frog produces sound by means of its vocal cords. The vocal cords consist of thin bands of tissue in the larynx (voice box), which lies between the mouth and lungs. When a frog forces air from its lungs, the vocal cords vibrate and give off sound. In many species, the males have a vocal sac, which swells to great size while a call is being made. Some species have a vocal sac on each side of the head. Others have a single sac in the throat region. Species that have a vocal sac produce a much louder call than do similar species with no sac.
Loading the player...Frog blows up its vocal sac
The life of a frog
Like all amphibians, frogs are cold-blooded or ectothermic—that is, their body tends to take on the temperature of the surrounding air or water. Frogs that live in regions with cold winters enter a state of inactivity in winter. Frogs in regions with harsh dry seasons may become similarly inactive. Some species spend such periods in burrows. They may keep themselves moist by producing a tough, mucus-filled cocoon of dried molted skin. Others spend the winter buried in mud at the bottom of a pond or stream, breathing through their skin. During such a period, a frog lives off energy stored in reserves of body fat. A few species can live through winters during which much of their bodies freeze. In these species, the cells produce a natural antifreeze that keeps vital parts of the body unfrozen, enabling the frogs to survive bitter cold.
Loading the player...Frog life cycle
Mating.
Most frogs that live in tropical and semitropical regions breed during the rainy season. In other regions, most species of frogs breed in spring or in early summer.
The majority of frogs, including most species that live on land, mate in water. The male frogs usually enter the water first. They then call to attract mates. Their call also helps direct other males to a pool suitable for mating. Each species has its own mating call. Naturalists can identify many kinds of frogs more easily by call than by appearance. Female frogs respond only to the call made by males of their own species. Individual differences in the mating call may help determine the female’s choice of mates. Males of some species also have a territorial call. This call warns other males of the same species that a certain area is occupied and that intruders are not welcome.
During reproduction in many frogs, a male grasps the female with his forearms and clings to her back. However, many other postures are known as well. In all such positions, together referred to as amplexus, the male fertilizes the eggs as they leave the female’s body.
Eggs.
The eggs of different species vary in size, color, and shape. A jellylike substance covers frog eggs, providing a protective coating. This jelly also differs from species to species. The eggs hatch within 3 to 25 days, depending on the species and the water temperature. Higher water temperatures speed up development, and lower temperatures slow it down. Among most species, a tiny, tailed tadpole or polliwog hatches from the egg.
Some species of frogs lay several thousand eggs at a time. But only a few of these eggs develop into adult frogs. Crayfish, fish, insects, and other water creatures eat many of the eggs. Even if the eggs hatch, the tadpoles face the danger of being eaten by larger water animals. In addition, the pond or stream in which the eggs were laid sometimes dries up, killing tadpoles.
Certain tropical frogs lay their eggs in rain water that collects among the leaves of plants or in holes in trees. Females in some of these species may return every few days to feed unfertilized eggs to their newly hatched tadpoles. 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 females of certain South American tree frogs carry the eggs in a pouch on their back. Males of another species, Darwin’s frog, carry the eggs in their vocal pouch until hatching, when tiny frogs emerge from the male’s mouth.
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.
Most 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 somewhat 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, making it possible for the animal to swim about to obtain food. Tadpoles use highly specialized mouth parts to eat algae, plants, decaying animal matter or other organic material, and microorganisms (tiny living things) that settle in the water. Some tadpoles eat frog eggs and other tadpoles.
In time, the tadpole begins to grow limbs. The hind legs appear first and are easily visible. The front legs develop next, but they remain hidden in the same body cavity that bears the gills until right before metamorphosis. At this time, the digestive system changes, enabling the frog to eat whole animals. The tadpole also loses its gills. Finally, a tiny frog, still bearing a stump of a tail, emerges from the water. Eventually, the animal absorbs its tail and takes on its adult form.
Some tadpoles are so small they can hardly be seen, but tadpoles of a South American species called the paradoxical frog reach more than 10 inches (25 centimeters) in length. A fully developed bullfrog tadpole may measure 6 to 7 inches (15 to 18 centimeters) long. It may take two or even three years for a bullfrog tadpole to develop into a frog. But among most species, the tadpoles change into adults within a few months. In a few species that breed in temporary ponds, this process may take less than two weeks.
Adults.
After a frog develops into its adult form, 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 3 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 predators as bats, fish, herons, raccoons, and snakes.
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. One frog from Brazil is known to eat berries. Otherwise, all adult frogs are strictly carnivorous (flesh-eating).
Most frogs have teeth only on their upper jaw, but many lack teeth altogether. Only a single species, Guenther’s marsupial frog from the Andes of South America, has teeth on the lower jaw. Frogs cannot chew, so the prey is swallowed in one piece. To aid in swallowing, the frog’s eyes sink through openings in the skull and force the food down the throat. Frogs absorb water through the skin only. With few exceptions, they cannot drink water through the mouth.
Kinds of frogs
Frogs make up the order Anura, one of the main groups of amphibians. Zoologists divide this order into a number of families of living species. The term toad is commonly applied to many unrelated species of frog. It is often used for frogs that tend to spend more time on land than water and that have a thicker, somewhat drier skin. The skin may be covered with many rough textured glands that superficially look like warts.
True frogs
Two well-known species are the northern leopard frog and European common frog. The northern leopard frog is a common species in North America. It is greenish in color with distinct dark brown spots. The European common frog has a brownish body. The North American bullfrog is a large true frog that may grow up to 8 inches (20 centimeters) in length.
Tree frogs
also live in most areas of the world, except Antarctica. They are also absent from most of Africa and Madagascar. The largest number of tree frogs live in tropical areas of Central America and South America. Most tree frogs measure less than 3 inches (7.6 centimeters) long and dwell in trees. Some North American tree frogs, called chorus frogs and cricket frogs, live mainly on the ground. Many species of tree frog are greenish or brownish, but some are among the most colorful amphibians of all.
Tree frog
Craugastorid frogs
live mainly in Mexico and Central and South America. These species lay their eggs under rocks and in crevices in rock piles. Tiny frogs hatch from the eggs without going through the tadpole stage. The barking frog is an example that lives on dry, rocky hillsides and cliffs in Texas and Mexico.
Narrow-mouthed toads
live throughout most tropical and subtropical regions and are especially common in New Guinea and Madagascar. As the name suggests, these frogs have an extremely narrow mouth. Many species, such as the sheep frog of North and Central America, live in burrows and eat ants and termites.
Spadefoot toads
live in Asia, Europe, North America, and northwestern Africa. The 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 dwell underground and are usually active at night. Several species live in dry regions of the United States and Mexico. These spadefoots may remain underground for months or even years at a time to stay moist. They breed following heavy rains, often laying eggs in temporary ponds. The tadpoles develop rapidly. If enough food is available, toadlets (tiny, developed toads) may emerge in only 12 days. The European spadefoot is known as the “garlic frog” because of its smell.
Poison frogs
live in Central and South America. They are known for the mild to lethal toxins that they excrete from their skins. Many but not all species are dazzlingly colored and are easily seen in the forest. The frog’s distinctive coloration warns predators that the animal is poisonous. Glands in the poison frog’s skin make the toxins using chemicals from ants and beetles that the frog eats. A few unrelated, non-toxic species of frogs have similar coloration, apparently mimicking the toxic species to ward off predators.
Glass frogs
live in Central and South America and are best known for the transparent skin on the bellies of many species. Various organs can be seen through the skin. Some organs may be covered with a distinct white tissue, including the beating heart and green-colored bones. Glass frogs are typically tiny and live along mountain streams. There, they attach their eggs to rocky waterfalls or the underside of leaves over water. The emerging tadpoles fall into the stream and develop there. In some species, the tiny male will remain with the eggs, aggressively protecting them from predators. 
Frogs and people
Frogs benefit people in many ways. They eat numerous kinds of insects, which might otherwise become serious pests. Frogs also provide us with food. The meaty hind legs of larger frogs are considered a delicacy in many countries. Frogs, especially the African clawed frog, are commonly used in research laboratories. Students dissect frogs to learn about anatomy, and medical researchers use frogs to test new drugs. The biochemistry of frog skin secretions is very complex, and researchers continue to test them for potential applications to human medicine.
Since the 1980’s, scientists have noted a worldwide decline of many frog species. Multiple factors are responsible for these declines, including pollution and habitat destruction. A particularly alarming threat has been identified in the form of a recently discovered species of disease-causing fungus. This fungus, known as amphibian chytrid fungus, is microscopic and infects the skin of frogs and other amphibians. The disease it can produce, amphibian chytridiomycosis, can cause severe declines in local populations and even extinction in some species.
