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Frog (animal)
I INTRODUCTION
Frog (animal), small, tailless animal with a squat body and long, powerful hind legs adapted for jumping. Frogs have large, bulging eyes and moist skin. They typically live on land but spend part of their time in the water. Most frogs develop from small, fishlike larvae called tadpoles or pollywogs that live in water. This life divided between water and land is typical of amphibians, a group of related animals that includes toads, newts, and salamanders as well as frogs.
Frogs and toads are very similar animals that together make up a group of amphibians called anurans. Many species of frogs have smooth skin and live near water, while toads typically have rough, warty skin and often live in drier habitats. Frogs usually have long legs and have teeth in their upper jaw, while toads have shorter legs and have no teeth at all. However, these distinctions are far from strict. Some animals commonly referred to as toads actually have characteristics that are more typical of frogs, and other animals have a mixture of froglike and toadlike characteristics. Scientists use the term frog to refer to any anuran, even those that are commonly called toads.
Scientists have identified over 4,000 species of frogs and continue to identify new species as they explore tropical rain forests and other remote parts of the globe. Frogs are an ancient group—the earliest known fossils of frogs date from the Jurassic Period (208 million to 144 million years ago). Frogs probably evolved from salamander-like animals that had long bodies, tails, and short hind and forelegs.
Frogs are of great importance to humans. Most frogs live on a diet of insects, and in many areas they help control populations of mosquitoes and crop-damaging insects. In turn, they may be a food source for humans—the legs of one type of frog are considered a delicacy in many parts of Europe. Frogs are also important in teaching and scientific research. Adult frogs are often used to teach students about the anatomy and physiology of vertebrates, or animals that have a backbone. Frog eggs, meanwhile, help scientists learn about embryonic development. Ecologists, scientists who study the Earth's living ecosystems, are interested in frogs and other amphibians because these animals are considered bioindicators. This means that the health of frog populations is thought to reflect the health of the ecosystem as a whole.
II RANGE AND HABITAT
Frogs live on all landmasses except Antarctica and some oceanic islands. They can be found in all but the most extreme habitats, such as the polar regions and the highest mountain peaks. Like many groups of animals, frogs reach their greatest diversity in the tropics (see Biodiversity). For example, more than 80 species of frogs live in the lowland rain forest of Ecuador. This number is nearly equal to the total number of frog species found in the entire continental United States.
A few frog species live in moist forest areas near small bodies of water, a habitat that provides a favorable temperature and keeps the frogs from drying out. Some frogs spend much or all of their time in water. These species typically have webbed feet that help them swim. Their skill at jumping and climbing enables frogs to exploit habitats that other groups of amphibians cannot reach, especially trees and bushes. Frogs that live in these habitats usually have expanded disks on the ends of their fingers and toes. These sticky disks help them cling to vertical tree trunks or branches.
In harsher environments, such as deserts, tundra, and mountains, frogs have developed special physical and behavioral features that enable them to survive. In dry regions, frogs usually have thick skin and live in tree holes or underground burrows. Tree-hole dwellers often have crests and flanges on their broad heads, which they use to plug up the tree holes after they back in. Burrowing frogs may have horny growths on their hind feet to help them dig through the ground. Some desert burrowers secrete a mucous cocoon around their body to prevent water loss while they are buried. The water-holding frog of the Australian outback emerges from its underground home to feed and mate only during the brief, infrequent rains.
III PHYSICAL CHARACTERISTICS
The smallest frog known is a species from Brazil that is just 8.5 mm (0.34 in) long (excluding limbs) and less than 30 g (1 oz). The largest is the West African goliath frog, which is 30 cm (12 in) long and weighs 3.2 kg (7.05 lb).
Frogs have a broad, flat head attached to a short, compact body. Their body structure is designed for jumping, their primary means of moving around on land. The lower segments, or vertebrae, of a frog's backbone are fused together to form a stiff rod called the urostyle, or coccyx. The urostyle and the frog's pelvic bones provide strength and stiffness to the rear of the body, where the jumping muscles attach to the skeleton. The long, muscular hind legs provide power for jumping. Frogs also use their strong hind legs for swimming, and some frogs use them for burrowing under ground. Frogs use their short, relatively thin forelegs to prop themselves up when they sit and to break their landing after they jump.
A frog's skin is usually moist and relatively thin, and lacks scales, hair, or other protective features. Glands in the skin secrete mucus to help keep the skin moist. Many frogs have gray, green, brown, or yellow skin that helps them blend in with their surroundings. Other frogs are turquoise, orange, red, blue, or other brilliant shades, and in many cases, this bright coloring serves as a warning to predators that the frog is poisonous. For example, the poison dart frogs of the South American rain forest, which come in a rainbow of bright colors, produce toxins that are potent enough to kill a human being. All frogs, not just those that are brightly colored, secrete a milky poison from glands in their skin to help them escape from predators. The poison produced by most kinds of frogs, however, causes no more than mild skin irritation for humans. Handling frogs and toads does not, as myth would have it, cause warts.
Frogs have sharp eyes that help them capture insect prey and identify predators or other dangers in their environment. The eyes of many frog species bulge out from the sides of the head, enabling them to see in nearly all directions and providing them with good depth perception, which helps them when capturing prey. Frogs also have a well-developed sense of hearing, which plays a role in locating mates and sensing predators. Behind each eye is a large disk called the tympanic membrane, an external eardrum that picks up sound waves and transmits them to the inner ear and then to the brain.
On land, frogs obtain oxygen primarily with their saclike lungs, using movements of the throat to push air into the lungs. They also obtain oxygen through their thin, moist skin, which is richly supplied with tiny blood vessels for this purpose. When underwater, frogs obtain all their oxygen through their skin.
Like other amphibians, frogs have a heart with three chambers—two atria, or receiving chambers for blood, and one ventricle, or pumping chamber. When a frog is on land, the ventricle pumps blood into two sets of blood vessels: one leading to the lungs, where blood picks up a fresh supply of oxygen, and the other leading to the body, where oxygen-rich blood is dispersed. When the frog is underwater and not using its lungs to breathe, the vessels leading to the lungs constrict and most of the blood goes to the body. This system provides efficient ways of delivering oxygen to the frog's body tissues regardless of whether the frog is in the water or on land.
IV FROG BEHAVIOR
Many aspects of frog behavior help the animals avoid conditions that are too hot, too cold, or too dry. Frogs are ectotherms (commonly referred to as cold-blooded), which means that their body temperature depends on the temperature of the surrounding environment. Few species can tolerate temperatures below 4 C (40 F) or above 40 C (104 F), and many species can survive only within a narrower range of temperatures. In addition, frogs' thin, moist skin offers little protection against water loss, and when on land the animals must guard against drying out. Many frogs are active at night because temperatures are cooler and humidity is higher than during the day. In seasonal environments, frogs may remain dormant for months at a time when conditions are inappropriate. For example, many frogs that live where the winters are cold spend the winter months buried in the mud at the bottom of a pond, and desert-dwelling species may burrow under the sand during the dry season or to escape extreme heat. Frogs do not drink, but they can replace lost water by absorbing it through the skin. A frog may sit in water or completely submerge to replenish moisture in its body. When a frog sits on moist ground, it absorbs water through the skin on its belly and the underside of its hind legs.
On land, frogs move from place to place by jumping, and some species, especially toads, move by crawling. Jumping enables frogs to move rapidly over land without leaving a scent trail, helping them escape from predators such as bats, herons, raccoons, snakes, turtles, fish, and even tarantulas. Although frogs escape from predators by jumping, they typically catch their own prey by simply sitting in one place and waiting. Most frogs feed on insects and other small invertebrates, such as worms, spiders, and centipedes. Aquatic frogs sometimes eat other frogs, tadpoles, and small fish. Larger frogs eat animals as large as mice or small snakes. When a frog sees that an insect or other prey animal is within range, it rapidly flips out its tongue, which is attached at the front of the mouth and catapults over the lower jaw. The tongue is coated with a sticky substance that holds on to the prey. Frogs generally swallow their prey whole. When a frog swallows, its eyes sink down through holes in the skull and help push food down the throat.
Frogs advertise their presence and communicate with other frogs using a variety of complex calls, including ribbets, croaks, and other sounds. They produce these sounds in much the same way as humans speak, by forcing air from their lungs over their vocal cords, located in the throat. Frog communication is particularly important during the mating season, when male frogs call to attract females (see Animal Courtship and Mating). The males of many species possess vocal sacs, expandable pouches of skin in the throat or on each side of the mouth. When the frog calls, the vocal sacs inflate and act as resonating chambers, amplifying the volume of the call. Each frog species has a distinctive call, enabling females to find an appropriate mate even when several different species are calling and breeding in the same area at once. If a female frog is clasped by a male that she does not want to mate with, she may give a call that tells the male to release her.
The breeding behavior of frogs is extremely variable. Some species congregate in large numbers around a pond, where they breed and lay their eggs in the water. Typically, after the female enters the water, the male frog positions himself behind her and grasps her by the waist. Once a suitable egg-laying site has been found, he releases sperm as she releases eggs, and fertilization—or union of the sperm and egg—takes place outside the body. Other species breed on land, and here again the male clasps the female from behind, releasing sperm as she lays her eggs on trees or other vegetation. Some male frogs are territorial during the breeding season, engaging in biting or wrestling combat with other males to compete for females.
Most frogs do not care for their young—the adults simply mate and abandon the eggs. A few frogs provide varying degrees of parental care, of which there are some spectacular examples. Either the male or the female may provide this care, depending on the species. Among some frogs one of the parents stays near the clutch of eggs and guards it from harm. The female marsupial frog takes her eggs with her, incubating them in a pouch on her back. In one Australian frog species, the female swallows the eggs and the young develop in her stomach and emerge from her mouth as tadpoles or froglets. In Darwin's frog, the male cares for the young, carrying the eggs and tadpoles in his vocal sac until they have matured into adults.
V LIFE CYCLE
Most frogs undergo a two-stage life cycle. Eggs hatch into fishlike young called tadpoles, which grow and eventually undergo metamorphosis, a change in body form, to become adults. Frogs have developed many variations on this common theme, and a few species provide striking exceptions to the rule. For example, frogs in the tropical genus Eleutherodactylus do not have a tadpole stage. Instead, tiny froglets hatch directly from the eggs. In the African toad, fertilization occurs internally, and the young are born alive.
Frogs often produce large numbers of eggs—up to 10,000 or more—surrounded by a jellylike coating that protects the eggs from predators and keeps them from drying out. Those that provide some degree of care for their young usually produce relatively few eggs. Species that breed in ponds typically lay their eggs in clumps or strings in the water. Those that breed in streams often attach their eggs to the undersides of rocks. Others construct foamy nests in the water or on vegetation near the water. Some frogs even attach their eggs to the undersides of leaves that hang over the water. When these tadpoles hatch, they drop into the water below. Many species lay their eggs in water trapped in tree cavities or cup-shaped parts of plants. In warmer climates, eggs may hatch within 1 or 2 days, while in colder environments they may take 30 to 40 days to hatch.
Tadpoles, which usually live in water, look somewhat like small fish. A tadpole has an oval body, gills for breathing, and a long, muscular tail with fins along the upper and lower edges for swimming. Tadpoles have two small eyes, one on each side of the head. A tadpole's mouth has a horny beak and rows of tiny, comblike teeth that the animal uses to scrape algae from underwater plants and bite off bits of plant material. In a few species, tadpoles eat the eggs and tadpoles of other frogs. Tadpoles pump water through their mouths, over the gills, and out through an opening called the spiracle, which is usually located on the left side of the body. The gills extract oxygen from the water. The tadpole stage lasts anywhere from a few weeks in species that lay eggs in temporary ponds to three years in the bullfrog. Typically, frogs spend a few months as tadpoles before metamorphosing into adults.
Metamorphosis involves radical changes in both external and internal body parts. The tadpole grows legs—the hind legs appear first—and resorbs its tail. It loses its gills and grows lungs, and the structure of the heart, digestive system, and skeleton changes. The horny beak and other mouthparts adapted for eating algae disappear and are replaced by the long, sticky, projectile tongue that helps adult frogs catch insects. Frogs reach reproductive age anywhere from several months to several years after metamorphosing. Most frogs probably live only a year or two in the wild, but bullfrogs live several years, and the African clawed frog has been known to live 35 years in captivity.
VI FROGS AT RISK
Since around 1980, scientists have reported startling declines in the populations of some species of frogs. These declines have occurred around the world, affecting frogs from California, Australia, the Andes of South America, and other parts of the globe. Some of these declines may reflect natural fluctuations, but scientists believe many frog species are in trouble because of environmental changes caused by humans. Pollution or destruction of frog habitat due to logging, industry, or development may be responsible for some population declines. However, frogs are also disappearing from relatively untouched habitats. The golden toad of Costa Rica and the gastric brooding frog of Australia have recently become extinct, despite the fact that they are native to still-pristine wilderness areas. The disappearance of this and other species has continued to baffle scientists, who fear that an exotic disease, such as a fungus that damages the skin, may have caused some of the more rapid declines. They also fear that global environmental changes such as acid rain, global warming, and increased levels of ultraviolet radiation due to the thinning ozone layer may threaten frogs even in areas far from human disturbance.
More recently, scientists have documented an alarmingly high occurrence of frogs with malformations such as missing or extra legs, abnormal webbing, and missing eyes. These malformed frogs were found first in Minnesota and have since been reported throughout the United States and Canada. The causes of these malformations are still uncertain but may include ultraviolet light, exposure to pesticides and other harmful chemicals, and parasites. The recent population declines and reports of malformed frogs have caused great concern among some scientists because frogs are considered bioindicator species. These scientists worry that whatever is causing problems for the frogs may ultimately threaten human health as well.
In 1991, the World Conservation Union (also known as IUCN) established the Declining Amphibian Populations Taskforce to coordinate research by some 1,200 individuals and agencies in 91 regions or countries. Various countries have also passed legislation to help protect frogs. For example, in the United States the Endangered Species Act, originally passed in 1973, provides protection for frog species that are judged to be in danger of extinction.
Scientific classification: Frogs are members of the order Anura in the class Amphibia. The bullfrog and the West African goliath frog are members of the family Ranidae and are classified as Rana catesbeiana and Conraua goliath, respectively. Poison dart frogs comprise several genera of the family Dendrobatidae. The African clawed frog belongs to the family Pipidae and is classified as Xenopus laevis. Marsupial frogs are members of the genus Gastrotheca in the family Hylidae. The extinct gastric brooding frog, Rheobatrachus silus, is classified in the family Myobatrachidae. Eleutherodactylus is a genus of frogs in the family Leptodactylidae. The family Bufonidae includes 380 species of toads, including the African toad, which is a member of the genus Nectophrynoides. Darwin's frog is in the family Rhinodermatidae and is classified as Rhinoderma darwini.
Reviewed By:
William S. Osborne
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Frog and Toad
Although toads and frogs are both amphibians, there are several basic anatomical differences between them. The smooth, moist skin characteristic of frogs covers long legs specialized for jumping and swimming. (It is these muscle-bound limbs that give the Edible frog (left) its name.) The skin of toads like the Green toad (right) is dry and covered by knobby glandular projections often referred to as “warts.” Toads have relatively stout, compact bodies. With relatively short legs, they lack the jumping ability and range of frogs.
Dorling Kindersley/Jerry Young
