True Frogs: Ranidae
TRUE FROGS: RanidaeMICRO FROG (Microbatrachella capensis): SPECIES ACCOUNTS
GOLIATH FROG (Conraua goliath): SPECIES ACCOUNTS
NILGIRI TROPICAL FROG (Micrixalus phyllophilus): SPECIES ACCOUNTS
POINTED-TONGUE FLOATING FROG (Occidozyga lima): SPECIES ACCOUNTS
BULLFROG (Rana catesbeiana): SPECIES ACCOUNTS
BROWN FROG (Rana temporaria): SPECIES ACCOUNTS
With nearly seven hundred species, the true frog family is very large. As in other big families, the true frogs come in many shapes and sizes, but they do have a few common features. All have teeth along the top of the mouth. Most of them have at least some webbing between the toes of the hind feet, and some have webbing all the way to the tips of the hind toes. In many species, the females are larger than the males, but the males have longer hind legs and more webbing between their rear toes. The males typically also have thicker front legs, which they use to hold onto the females during mating.
Most true frogs are shades of greens or browns and blend in well with their surroundings. Those that live in the water, such as Roesel's green frog and Indian tiger frog, are commonly green to olive green in color, which matches well with their homes. Often, species that live in forests, like the wood frog and Beddome's Indian frog, are tan or brown like the dead leaves that cover the forest floor. Most true frogs have rather stocky bodies. The African bullfrog is especially pudgy-looking. Some, like the leopard frog and pickerel frog, have more slender bodies. Many members of this family have long and strong hind legs for leaping. This includes the leopard, pickerel, and green frogs that are found in North America, as well as many others. Some, like the large African bullfrog, have shorter hind legs.
Many of the frogs in this family have a dark, horizontal bar through the center of each eye. Some also have a dark, vertical bar through the eye. In addition, many have a noticeable eardrum on each side of the head behind the eye. In a few species, like the green frog, the size of the eardrum can help tell a male from a female. In this species, the male's eardrum is much larger than the eye, while the female's is about the same size or smaller than the eye.
Many true frogs are between 1.6 and 3.3 inches (4.1 to 8.4 centimeters) long from the tip of the snout to the end of the rump. The smallest frogs in this family, however, are 0.4 inches (1 centimeter) long. These include several tiny African species. The largest members of this family, including the goliath frog, can reach more than 12.2 inches (31 centimeters) long.
True frogs live in much of the world, including most of North and Central America, the north end and parts of central South America, most of Europe and Asia, much of Africa, and Australia. Some members of the family are also found on islands in the ocean. Some populations, like those in much of Australia, were introduced to these areas by humans and previously did not live there.
Many species live near water, such as a pond, quiet stream, or marsh. A few, like the goliath frog, make their homes in fast-moving rivers and rapids. Other true frogs, however, spend most of their lives away from water in forests or grasslands, and only return to ponds or wetlands once a year for breeding. Of these land-living frogs, some do not return to the water at all and instead lay their eggs in moist places on land.
Most of the true frogs are active at night and eat insects and other invertebrates (in-VER-teh-brehts), which are animals without backbones. The larger members of this family have more variety in their diets and will eat tadpoles of other species and sometimes their own, as well as animals like snakes, birds, and small mammals. Some of the true frogs hop about looking for food. Others are ambush hunters, which means that they sit still and wait for an insect or other animal to walk past. With a simple flick of the tongue or grasp of the mouth, they capture the victim and eat it whole.
BEHAVIOR AND REPRODUCTION
Although most members of this family are active at night, some that live next to lakes and ponds, or in cooler areas, are out and noticeable during the daytime. They sunbathe, or bask, by sitting in a warm spot on land or, if they are in the water, by floating in the warm, top layer. Some commonly seen daytime-basking frogs in North America include the green frog and leopard frog.
Those true frogs that live in moist forests in warm climates remain active all year. Others do not. Some live in dry areas and have to find ways to survive the weather. The African bullfrog, for example, makes its home in dry regions of southern Africa. It becomes active and breeds during the rainy season, but when the ground dries out, it buries itself underground. It then sheds a few layers of skin that it wears like a watertight coat to keep its skin from losing too much water. This skin cocoon stays around its body while the frog rests during a period that is known as estivation (es-tih-VAY-shun). The frog remains in its cocoon until the next rainy season arrives.
Frogs that live in places with cold winters also enter a resting period, known as hibernation (high-bur-NAY-shun). Both hibernation and estivation are long resting periods, but one happens when the weather is cold, and the other when the weather is dry. The hibernating frogs typically bury themselves in the ground or in mud at the bottom of their pond or wetland and stay there until the temperatures warm the following spring. A few species, like the wood frog of North America, are able to freeze solid in the winter and recover the next spring to live another year.
Most of the species in this family avoid predators by restricting their activity to the dark of night or by remaining still and blending in with the background. Since many of them live near the water, they also have the option of leaping and then diving down to make a fast escape if predators come too close. A few, like the African bullfrog, will stand their ground. They will nip at predators that approach them or their young.
Most of the frogs in this family breed during one season a year. Those that live in dry areas mate during the rainy season. Others that live in climates with cold winters start their breeding seasons when warmer spring temperatures arrive. Some breed in early spring, and others in late spring. Those that live in warm, tropical areas may breed more than once a year, often following heavy rains. Often, true frogs mate together in large groups. In the wood frogs, for example, dozens of males will hop over to the water and begin calling all at once. This type of group calling is called a chorus (KOR-us). With their quacking calls, the wood frog chorus sounds something like a large group of ducks. They, like many other species of true frogs, are explosive breeders, which means that they breed over a very short period of time. All of the wood frogs in a population, for instance, may mate over just seven to fourteen days. The male true frogs may have one or two balloon-like vocal sacs in the throat area. These fill with air and deflate as the frog makes his call. Many species, like the green frog and leopard frog, have one large vocal sac. The wood frog is one of the species with two smaller vocal sacs. They typically both inflate at the same time—one on each side of its throat.
Like females of other frog species, female true frogs follow the males' calls. Different species have different calls. The green frog, for instance, has a short "gung" call, while the leopard frog's call is more of a snoring sound. In many species, including the bullfrog, males may wrestle with one another for a good calling area. Once males and females are together, one male and one female typically pair off, the male scrambles onto her back and hangs onto her while she lays her eggs. The male in most species clings to the female by gripping near her front legs. Depending on the species, the female may lay a few or many eggs. The female Penang Taylor's frog, for instance, lays five to thirteen large eggs; the female Sanderson's hook frog lays twelve to seventeen; the female African bullfrog lays three to four thousand; the female Roesel's green frog lays two to six thousand; and the bullfrog female can lay up to twenty thousand eggs. In some species, like the African bullfrog, one male may mate with more than one female.
The wood frog of North America survives the cold of winter by freezing solid and remaining that way until it thaws out in the spring and hops away. Most other animals would die if frozen in this way. The wood frog prepares for winter by scooting under a pile of leaves when the chilly weather arrives in the fall. Then its body starts to make a sugary substance called glucose (GLOO-cose) that acts as antifreeze in its heart and other major organs and protects them from damage. Even though its heart does not beat, and the frog does not breathe all winter long, it continues to live to face another year.
In most species, the females lay their eggs in the water, and the eggs hatch into tadpoles. The tadpoles may turn into froglets in a few weeks or, in some cases, in a few years. Bullfrog tadpoles are an example. They can survive as tadpoles for up to four years and even hibernate just as their parents do. In a few species, like Penang Taylor's frog, the female lays her eggs in a moist place on land, and the eggs hatch right into froglets, skipping the tadpole stage. In either case, eggs typically hatch in a matter of days to weeks. In most true frogs, neither parent provides any care for the young once the eggs are laid. In others, however, the parent may remain nearby to make sure that predators do not eat the young and/or to help keep the eggs moist if they are laid on land. The female Sanderson's hook frog even returns to her eggs every night to cover them with her body. In the African bullfrog, it is the male that watches over his young. He will bite at any intruder who comes close to his eggs or tadpoles, even if the intruder is as large as a lion or a person. If the weather becomes dry very quickly and the tadpoles are trapped in a small puddle away from the main pond, he may also dig a path through the mud to the pond so the tadpoles can swim from the puddle to the deeper water of the pond, where they continue their development.
TRUE FROGS AND PEOPLE
People from many countries, including the United States, eat frog legs. The legs usually come from large true frogs, often bullfrogs that have been captured from the wild. Some Asian and African people also make frog soups and other meals out of entire frogs and tadpoles. In addition, some people in different parts of the world use certain parts of frogs as medicines.
The World Conservation Union (IUCN) lists two of the species as being Extinct, which means that they are no longer in existence; twenty species that are Critically Endangered and face an extremely high risk of extinction in the wild; fifty-nine species that are Endangered and face a very high risk of extinction in the wild; eighty-four that are Vulnerable and face a high risk of extinction in the wild; fifty-nine that are Near Threatened and at risk of becoming threatened with extinction in the future; and 132 that are Data Deficient, which means that scientists do not have enough information to make a judgment about extinction threat.
The two Extinct species are the Las Vegas leopard frog, which is also known as the Vegas Valley leopard frog, and another species known only by its scientific name of Nannophrys guentheri. The Las Vegas leopard frog was only found in a few places north of Las Vegas Valley in Nevada, but it has not been seen since 1942. Ecologists believe that it died off when people rerouted water from the frog's breeding areas to the growing city of Las Vegas. A few small areas with enough water still remain, but people introduced bullfrogs to those areas. Since bullfrogs eat other, smaller frogs, any remaining Las Vegas leopard frogs would probably have been gobbled up. No one has seen the other Extinct species, Nannophrys guentheri, since the first one was seen in Sri Lanka more than a century ago.
One of the twenty Critically Endangered true frogs is the dusky gopher frog of the United States, which is also known as the Mississippi gopher frog. Although this species once lived in parts of Louisiana, Mississippi, and Alabama, it now only survives in a small area called Glen's Pond, which is in Mississippi's Desoto National Forest. The last members of this species were seen in Alabama in 1922 and in Louisiana in 1967. In 2001 fewer than one hundred adult frogs were still alive in Glen's Pond. Ecologists mainly blame the drop in numbers on two diseases caused by fungi. One of the fungi, known as chytrid (KIT-rid) fungus, has also killed many other frogs around the world. A Gopher Frog Recovery Team is now watching over the frog and its habitat and is trying to treat the tadpoles infected with the fungus.
In addition to the at-risk true frogs noted above, the U.S. Fish and Wildlife Service also considers four to be Endangered or Threatened. These include the California red-legged frog and the Chiricahua leopard frog, which are Threatened or likely to become endangered in the near future; and the Mississippi gopher frog described above and the mountain yellow-legged frog, which are Endangered or in danger of extinction throughout all or a significant portion of their ranges.
Physical characteristics: As its name suggests, the micro frog is tiny. In fact, they are some of the smallest frogs in the world. Adults reach just 0.4 to 0.7 inches (1.0 to 1.8 centimeters) long from the end of the snout to the back of the rump. Their hind legs are fairly short and end in long-toed feet. The front legs are short and thin. The toes have some webbing between them, but the very long fourth toe on each rear foot is mostly free of webbing. These frogs come in several different pale or dark colors, including green, tan, reddish brown, gray, and black. A dark stripe runs from the eye to the front leg. Many frogs have a noticeable, but thin, light-colored or greenish line that starts at the snout and continues over the top of the head and down the back to the rump. Some micro frogs also have dark patches low on their sides and dark-colored speckles on top of the back and head. The back and top of the head have a few, small, scattered warts. The underside of the frog is smooth and dappled with black and white. Sometimes the underside is pale-colored without the black-and-white pattern. The eyes are large and brownish, and the snout is short and slightly narrower toward the front. The male and the female look very much alike. The male, however, has a large vocal sac that covers half of his underside. The vocal sac is usually not noticeable unless the male is calling. To make his call, he blows up the vocal sac to a size almost as big as his entire body.
Habitat: It makes its home in rotting plant roots of shrub-filled woodlands near small pools that fill with water only during part of the year.
Diet: Scientists are not sure what it eats, but if it is like other small frogs in its family, it probably eats small insects or other invertebrates.
Behavior and reproduction: Scientists know little about this frog outside of its breeding season, which runs from June to July. When they are ready to breed, the males sit in the water along the edge of the pool. There, hidden among plants that grow in the shallow water, they call with about half of their bodies above the surface. Each male calls with a one-second long scratchy noise that sounds like "tschik," which they repeat five or six times. Females follow the calls, and a male and female pair off. The female lays about twenty eggs, each of which is small and coated with gel. The eggs stick together in a clump and attach to underwater plants. The eggs hatch into tadpoles. They continue to grow in the water for the next six or seven months. In December, when they have reached about 1 inch (2.5 centimeters) long— 70 percent of which is tail—they turn into baby frogs.
Micro frogs and people: People rarely see this tiny frog.
Conservation status: The IUCN considers this frog to be Critically Endangered, which means that it faces an extremely high risk of extinction in the wild. It lives in a very small area and does not do well around humans. People are, however, moving closer to the frog to construct homes and enlarge their farms. In addition, they are draining water from the wetlands where the frogs breed and have introduced new plants, which are also using up water. ∎
Physical characteristics: True to its name, the goliath frog is huge. It is the largest frog living on Earth today. An adult can reach a whopping 12.6 inches (32 centimeters) long from snout to rump. One frog can weigh 7 pounds (3.25 kilograms). The frog has a wide, flattened body that blends into its also-wide head without a noticeable neck. Its head is shaped like a triangle with a blunt point at the end of the snout. The frog's back, head, and the upper surface of all four legs are dark gray, sometimes a bit brownish or greenish, and covered with tiny bumps. Faint dark bars and/or spots sometimes show on the legs and lips. The underside is a lighter color, often appearing a greenish tan. The hind legs are long. The front legs are shorter, but thick. The toes on the front feet have a bit of webbing at their base but not out to the ends. The hind toes have full webbing all the way to their tips.
Geographic range: It lives in a small part of west-central Africa from southern Cameroon to parts of Guinea.
Habitat: The goliath frog is at home in rapids and other fast-moving parts of rivers.
Diet: Scientists are not sure what it eats. Given its enormous size, many types of food are possible.
Behavior and reproduction: Scientists know little about this frog's behavior outside of the breeding season. At that time, the males begin calling, but they do it in a way that is different than most other frogs. Most male frogs suck air into vocal sacs and blow it out to make their calls. The goliath frog and other closely related species have no vocal sacs and instead hold the mouth barely open and make a long whistling noise. Females follow the whistling to the males. One female can lay several hundred eggs at a time. Each egg is very small, about 0.14 inches (3.5 millimeters) in diameter, and sticks to plants growing in rocky areas of the river rapids. The eggs hatch into tadpoles, which can grow to 1.9 inches (4.7 centimeters) long over the next eighty-five to ninety-five days. They then turn into froglets.
Goliath frogs and people: Local people often hunt this frog for food by searching for it from boats and using a gun to shoot at it. Once they have wounded or killed it, the hunters leap into the water to snatch up the frog. New traps for capturing the frogs are making the hunters even more successful. The hunters may either eat the frogs themselves or sell them to markets. Some people also capture goliath frogs alive to sell in the pet trade, to zoos, or to people who hold frog races.
Conservation status: According to the IUCN, this species is Endangered, which means that it faces a very high risk of extinction in the wild. In the fifteen-year period from 1989 to 2004, the number of goliath frogs dropped by more than half. Hunting them for meat and collecting them for the pet trade are the main reasons for the fall in numbers. In addition, the frog's forests are also disappearing as people cut down trees, farm the land, and construct buildings. These activities are also allowing soil to run downhill and muddy up the streams where the frog breeds, and this may hurt the tadpoles. Some people believe that ecologists should begin breeding the frogs in captivity to make sure the species survives into the future. ∎
Physical characteristics: The typical Nilgiri tropical frog is a greenish brown, smooth-skinned frog with darker patches scattered on its back and dark bands across its front and back legs. Some pink color shows on its underside beneath the legs and toward the rump. It has a ridge of skin running down each side from the snout to the rump. Its hind legs are fairly long, and their toes are fully webbed. The toes on all four feet have small, rounded tips. Males and females look much alike, except that the males develop rough pads during the mating season. Adults reach about 1.25 inches (3.175 centimeters) long from snout to rump.
Geographic range: It lives in southern India.
Habitat: The Nilgiri tropical frog makes its home in moist, humid forests on hills from about 984 to 4,593 feet (300 to 1,400 meters) above sea level. The forests all have thick layers of dead leaves and other bits of plants lying on the ground. The frog appears to live only in forests that humans have not logged or otherwise changed.
Diet: Scientists are not sure what it eats.
Behavior and reproduction: This frog is still mostly a mystery. Other than the tadpole's appearance, scientists know little else about it. The tadpole is long with a slender tail and a mouth that opens on the bottom rather than on its front, as is the case in many other tadpoles.
Nilgiri tropical frogs and people: This frog does not survive well in disturbed forests, and people are doing just that by logging trees from woods where the frog lives.
Conservation status: The Nilgiri tropical frog is only known from one national park and two reserves, but it may live in areas between these three spots. Because the frog's home areas are small and separated from one another, and its forests are being logged, the IUCN has listed the Nilgiri tropical frog as Vulnerable, which means that it faces a high risk of extinction in the wild. It is protected by the government of India. ∎
Physical characteristics: The pointed-tongue floating frog goes by several other names, including floating spotted frog, java frog, green puddle frog, and pearly-skin puddle frog. It has a fairly plump body and a short head that narrows to a somewhat-pointed snout. It has a pointed tongue. Many small bumps cover its back, head, and legs. It is usually dark brownish green, but is sometimes pinkish brown. Some of the frogs have a thin stripe down the middle of the back. Although the webbing of their feet is thin and may be difficult to see, it is there and stretches fully between their pointy toes. Males and females look much alike, except that the males develop rough pads during the mating season. Adults reach about 1.5 inches (3.9 centimeters) long from snout to rump.
Geographic range: It lives in southeastern Asia, including southern China, India, Cambodia, Vietnam, Indonesia, and Malaysia.
Habitat: It is believed to spend most of its time in the water of slow-moving streams, marshes, wet rice paddies, and other wetland areas. These areas are often surrounded by grasslands.
Diet: Scientists are not sure what it eats.
Behavior and reproduction: Scientists know little about its behavior outside of the breeding season. During breeding, males call with two short notes. Females answer the calls, and male and female pairs form. The male climbs onto the female's back and grips her by her front legs. She then lays her small eggs. The eggs hatch into long, pointy-snouted, small-mouthed tadpoles. The tadpoles grow in the water until they turn into froglets.
Pointed-tongue floating frogs and people: People rarely see this frog.
Conservation status: The IUCN does not consider this frog to be at risk. It lives over a large area and is usually quite common. ∎
Physical characteristics: The largest frog in North America, the bullfrog can grow to 8 inches (20.3 centimeters) and weigh more than
3.3 pounds (1.5 kilograms). It may be green, brown, or greenish brown, sometimes marked with dark spots on its back and legs. It has long, strong hind legs with toes that have full webs between them. It has a pair of large eardrums showing on each side of the head. Males and females look much alike, except the male's throat is yellowish and the female's is cream-colored, and the male's eardrum is much larger than the eye, while the female's is about the same size as the eye. The bullfrog looks similar to another species, known as the green frog (Rana clamitans). The green frog, however, is smaller and has a fold of skin running down each side of the back. The bullfrog does not have these long folds. Instead, it has a smaller fold that curls from the back of the eye around the eardrum.
Geographic range: The bullfrog is an eastern North American species that lives in northern Mexico, the United States, and southern Canada. Over the years, it has also been introduced to other places in the world, where it does very well including parts of Central and South America, the West Indies, several countries in Europe and southeastern Asia, and some ocean islands, including Hawaii.
Habitat: It makes its home in almost any large, calm body of water, including ponds, bays of the Great Lakes, slow backwaters of rivers and streams, and marshes that are filled with water all year long. Adults are not found in wetlands that dry up for part of the year. In Hawaii, some bullfrogs can even survive in somewhat salty water.
Diet: Bullfrogs hunt by ambush, which means that they sit still and wait for their meals to come to them. Meals may include other frogs, including bullfrog tadpoles and other, younger bullfrogs; various animals, such as snakes, fish, ducklings, and other birds; and many different kinds of invertebrates, like insects, worms, spiders, and snails.
Behavior and reproduction: It usually stays along the edge of its water body, sitting among reeds and other plants that are in the water or just on shore. This frog is active during warm weather. When the cold autumn temperatures arrive, it buries itself in the muck at the bottom of the water and enters hibernation until warm weather returns in the spring. During the breeding season, which runs from spring to mid-summer, each male will defend his piece of shoreline against other male bullfrogs by first making a short warning call, and if that does not work, by pushing the male frog, sometimes even getting into wrestling matches. Males make loud, deep calls, which some people describe as sounding like a slurred "jug-o-rum." A male and female pair may mate at the calling site or move a sort distance away. A female can lay three thousand to twenty thousand eggs, each of which measures only 0.05 to 0.07 inches (1.2 to 1.7 millimeters) across. The eggs hatch within a week into small tadpoles. Unlike the tadpoles of most other species, which turn into froglets within a few months, bullfrog tadpoles may wait from two to four years and grow to 6.7 inches (17 centimeters) long before making the change.
Bullfrogs and people: Many high school students are familiar with this species as the frog they dissect in biology class. It also has other uses. This frog is captured for food, and its legs are served at restaurants across the United States and elsewhere.
Conservation status: This is a very common frog and is not considered to be at risk. Instead, it has become a pest species in many areas of the world where humans have introduced it. This is because the bullfrog not only competes with other species for their food but also eats the other frogs. ∎
Physical characteristics: The brown frog is sometimes called the European common frog or the grass frog. It is typically a tan frog, but some are darker brown, brownish green, gray, or black, and a few are tinted with red or yellow. Warts are scattered on its back, and these usually sit in small, dark brown blotches. The frog also has dark brown bands on its hind legs. Similar bands on its front legs are usually broken and fainter. It has a light-colored thin fold of skin down each side of its back and a dark patch of color behind each eye. Its head narrows toward the front to a somewhat pointy snout. The front feet are unwebbed, but the hind toes have a good deal of webbing between them. The underside of the frog is usually off-white or yellowish white in males and yellowish white to orange in females. During the mating season, the male's throat becomes blue-colored. Adults grow to 2.4 to 3.7 inches (6.0 to 9.5 centimeters) long from snout to rump.
Geographic range: It lives throughout Europe.
Habitat: Adults mainly live along the forest floor or in grasses for most of the year. In the north where temperatures are cooler, they stay in lowland areas, but they may live high in mountains in the south, as much as 6,562 feet (2,000 meters) above sea level. The tadpoles develop in wetland areas.
Diet: Although this frog is found in much of Europe and is fairly common, scientists still are unsure what it eats.
Behavior and reproduction: In daytime it stays out of sight in damp areas. It becomes more active at night, when it does much of its hunting. It may also become active on rainy days. In northern climates where the weather turns cold in the winter, the brown frog hibernates at the bottom of a pond or under piles of rotting leaves and plants. As soon as the spring sun has melted the snow and ice from the ground and the frogs awaken, the breeding season begins. Males gather at the water and start calling, sometimes wrestling with one another over the females. To mate, a male climbs on the back of a female. The pair may remain together in this piggyback position for a few days. Each female lays one thousand to four thousand small eggs in shallow water. In about two weeks, the eggs hatch into tadpoles, which grow to as much as 1.77 inches (4.5 centimeters) long before they turn into froglets.
Brown frogs and people: They are common in country gardens and other places near humans. Some people in Europe eat these frogs.
Conservation status: Even though this frog is not considered at risk, some populations of it have become small because of over-collecting for various purposes, such as their use as food or their sale in the pet trade. Ecologists are also concerned about the effects of pollution on the frogs and about the draining of their breeding areas. ∎
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