CLAWED FROGS AND SURINAM TOADS: Pipidae

PHYSICAL CHARACTERISTICS

With their flat bodies and their wide and fishlike heads, the clawed frogs and Surinam toads are an odd-looking bunch. The head is flat in some species and shaped like a wedge—taller in the back and tapering down toward the front—in others. They have tiny eyes on the top of the head, but they do not have tongues. Their eardrums do not show on the sides of their heads, as they do in many other frogs. Another unusual feature is the line of stitchlike marks that run down each side of the body from the head to the rump. The marks are not actually stitches but allow the frog to feel the movements made by other animals in the water. This line of marks is known as a lateral (LAT-eh-rul) line system. While such a system is common in tadpoles and in fishes, it is unusual in adult frogs. The lateral line, which senses vibrations in the water, is helpful in finding prey.

Clawed frogs and Surinam toads have long hind legs with large, fully webbed feet. The webbing may match their foot color, or it may be a different color, such as the orange-yellow of the Müller's plantanna. The forelegs of clawed frogs and Surinam toads are much smaller than their hind limbs, and the thin toes on the front feet in most species do not have any webbing between them. The only members of this family with front toe webbing are the dwarf clawed frogs, which fall into the groups known as Hymenochirus and Pseudhymenochirus. All except two species, including the Surinam toad, have three claws on each foot. The claws are the second, third, and fourth toes. The outside two toes on each hind foot are clawless. A typical species in this family has a tan, greenish brown, or gray back, usually with dark spots or markings, and a lighter colored underside with dark markings. Depending on the species, they may either have bumpy or smooth skin. The adults of some species grow to 0.8 to 1.2 inches (2 to 3 centimeters) long from the tip of the snout to the end of the rump, but others can reach as much as 4.1 to 6.7 inches (10.4 to 17 centimeters) in length.

None of the 30 species in this family has vocal sacs. In most other species of frogs, the vocal sac looks like one or two bubbles located below the chin that blow up with air and deflate when the frog makes its call. The clawed frogs and Surinam toads do not even have vocal cords, which are the structures inside the throat that most animals, including humans and other mammals, use to make noises. Instead, these frogs have two disks in their throats that move back and forth to produce clicking sounds. Since these frogs do their clicking underwater, the sound travels through the water as a vibration, rather like an underwater ripple. Another frog can hear the clicks through a different disk that sits under its skin and on the side of its head. This disk picks up the vibration and passes it along to the inner ear, which is the part of the ear located inside the head, and the frog hears the click.

GEOGRAPHIC RANGE

Clawed frogs and Surinam toads live in tropical parts of South America, but not in mountains, and in the central and southern region of Africa, which is known as sub-Saharan Africa. Humans have brought them to other places, including the United States, the United Kingdom, and other parts of Europe and South America.

FROGS IN SPACE

Four female frogs soared into orbit in 1992 on board the Space Shuttle Endeavor. Their trip was part of an experiment to see whether their eggs could live and grow normally in a gravity-free environment. Scientists treated the females with a chemical that triggered them to lay their eggs. They then added a male frog's sperm to the eggs to begin development. Through experiments like this one, they hope to learn how difficult it may be for humans to live and reproduce in space.

HABITAT

Members of this family live in many different watering holes including mucky swamps, small pools of water that dry up during part of the year, large ponds that are filled all year long, and slow-flowing rivers and streams. They rarely leave the water. If their pool dries up, they typically burrow into the still-wet muck at the bottom and wait for the rains to return.

DIET

Many of the tadpoles are filter feeders, which means that they suck in water and strain out bits of food that were floating in it. The tadpoles of the dwarf clawed frogs actively hunt down and eat insects and other invertebrates (in-VER-teh-bre-hts) or animals without backbones that they find in the water. Adult clawed frogs and Surinam toads eat insects, fishes, occasionally their own tadpoles, as well as mammals and birds that may fall into the water. Since they have no tongues, they must lunge at prey and grab it with their mouths. Some use their front feet to stuff their catch farther into their mouths and/or use the claws of their hind limbs to shred their catch before swallowing it down.

BEHAVIOR AND REPRODUCTION

Clawed frogs and Surinam toads spend much of their time floating in the water with their legs held out from the sides of their bodies. Their dark colors blend in with the water, which makes them difficult for predators to see. They are also quite skittish. The common plantanna, for example, will dive to the bottom of its watering hole as soon as it feels the least bit threatened. Clawed frogs and Surinam toads almost always stay in the water, but sometimes, on very rainy nights, these frogs may leave the water and move short distances from one pond to the next. If they happen to live in a pool of water that dries up during part of the year, clawed frogs and Surinam toads typically bury themselves in the muddy bottom and wait for drenching rain to wet the ground again. This underground waiting period is called estivation (es-tih-VAY-shun). During this time, they do not eat and instead live off fat and other stored energy in their bodies. They may estivate for several months at a time.

Although scientists have not studied these water-loving species very closely, they believe the frogs may mate at any time of year, as long as a heavy rain has soaked the land. Both males and females have the throat disks that allow them to make underwater clicking noises. Each species can make three to six different types of clicks, at least one of which is used to signal that mating time has started. Males climb onto the backs of the females to mate. If a male accidentally climbs onto the back of another male, the second male will click differently to tell the other to get off. As the males and females mate, the females of most species drop their eggs in the water. The eggs hatch into tadpoles, some of which have tiny bits of flesh dangling from the edge of the mouth. These fleshy bits, called barbels (BAR-bulls), are feelers.

Female Surinam toads lay and raise their eggs in a more bizarre way. She flips over while she is laying her eggs, and the eggs settle onto her back, where they stick. Her flesh then swells up around the eggs, turning her back into a sponge-like cradle for them. Depending on the species, the eggs may hatch into tadpoles, or they may skip the swimming tadpole stage and hatch right into froglets. Scientists know this type of back cradle occurs in all but one of the Surinam toads. This is the Myers' Surinam toad, and scientists still are not sure how and where its eggs hatch.

CLAWED FROGS, SURINAM TOADS, AND PEOPLE

People rarely see these frogs in the wild, but they are still very important creatures. Scientists have discovered some unusual chemicals in their skin that may be useful in treating illnesses or in preventing infections. One of these substances, first found in the skin of the common plantanna, is called magainin. Magainin is a piece of protein known as a peptide that fights both germs and fungi. Scientists have started to test this peptide and other compounds like it for use on such things as bandages to help cuts and other wounds heal faster. In addition, scientists use these frogs in the laboratory to study the development of their eggs. Common plantannas carry their see-through eggs on their backs, which provides a good view of the young as they grow and develop inside the egg.

CONSERVATION STATUS

Of the 30 species in this family, the World Conservation Union (IUCN) considers one to be Critically Endangered, which means that it faces an extremely high risk of extinction in the wild; two to be Endangered and facing a very high risk of extinction in the wild; and one to be Near Threatened, which places it at risk of becoming threatened with extinction in the future. It also views seven other species as Data Deficient, a category that means the IUCN does not have enough information to make a judgment about their threat of extinction.

The Critically Endangered species is the Lake Oku clawed frog, which lives in a single lake in western Cameroon. The lake currently has no fish in it to prey on the frogs, but conservationists fear that an introduced fish might find its way into the lake and possibly wipe out the entire frog species. The two Endangered species are the Myers' Surinam toad and Gill's plantanna (also known as the Cape clawed toad or Cape plantanna). Myers' Surinam toad lives in a small area in Panama, and Gill's plantanna lives in a tiny part of southwestern South Africa. Habitat loss is a threat to both species. In addition, water pollution appears to be hurting the Myers' Surinam toad.

COMMON PLANTANNA (AFRICAN CLAWED FROG) (Xenopus laevis): SPECIES ACCOUNTS

Physical characteristics: The common plantanna goes by several different common names, including African clawed frog and clawed toad. It has a flat head and body with long and strong hind legs. Its back and the top of its head are dark-colored, usually gray to olive-brown and sometimes are marked with dark, occasionally orangish, spots. Its underside is lighter colored and may be off-white, light gray, or grayish yellow, sometimes with faint, gray speckles. A thin row of what look like stitches run down each side of the otherwise smooth back from behind the eye to the rump. Each row holds 23 to 31 "stitches." The plantanna has two tiny eyes on top of its wide head and a tiny bit of flesh, called a tentacle, under each eye. It has no tongue. True to its name, it has little black claws on three toes of each hind foot. The feet are quite large and have gray webbing between and to the tips of the long toes. Sometimes the webbing has a little yellow or orange color to it.

Males and females of this slippery-bodied frog look nearly alike, except that males are smaller. A male grows to about 1.8 to 3.8 inches (4.6 to 9.7 centimeters) long from the tip of the snout to the end of the rump, while females can reach 2.2 to 5.8 inches (5.6 to 14.7 centimeters) long.

Geographic range: The common plantanna is an African frog that lives as far south as South Africa and north to Kenya, Uganda, the Democratic Republic of Congo, and Cameroon. In addition, people have introduced them to many other nations, including England, Germany, Chile, and the United States.

Habitat: The common plantanna is a water-loving species. It can live in all types of water from fast-moving rivers to calm ponds. It can even survive in mucky pools and swamps and in somewhat salty water. Although its native African habitats do not cool much in the winter, it has held up well in places that have winters cold enough to freeze the tops of ponds.

Diet: Tadpoles eat by straining little pieces of algae (AL-jee) and other tidbits from the water. Algae are tiny plantlike organisms that live in water but have no true roots, stems, or leaves. Once the tadpoles change into froglets, their diet switches to insects and other invertebrates they find in the water. Adults also eat young, or larval (LAR-vuhl), mosquitoes and other insects, sometimes leaping out of the water to nab a flying insect, and will eat larger things, such as fish or birds and mammals that fall into the water. Sometimes they even eat their own eggs and tadpoles.

Behavior and reproduction: Common plantannas stay in the water for almost their entire lives, only coming onto land now and then at night. People most often see them floating at the water's surface, with legs outstretched, and only the top of their heads out of the water. If their pond or swamp dries up, they will dig down into the mud, hind end first, and bury themselves until the rains return.

In addition to signaling the end of estivation for some frogs, the rains combine with warm weather to trigger the mating season for the entire species. Unlike most other types of frogs, both the males and the females call. Their call, which they make underwater, sounds like a buzzing tap. The females lay about one thousand tiny tan eggs, which stick to underwater plants and rocks.

Common plantannas and people: Though it seems strange now, until the 1940s, people turned to these frogs to learn whether a woman was pregnant. To do it, they used a hypodermic needle to suck up a little of the woman's urine and then inject it under the skin of the female frog. If the woman was pregnant, the hormones in her urine would spark the frog to start laying eggs. If the woman was not pregnant, the frogs laid no eggs. Medical professionals now find out if a woman is pregnant through other tests that do not involve frogs.

Nowadays, this species has another role in medicine. One chemical in its skin kills bacteria and may be a useful antibiotic, while another may be helpful in explaining how the human brain works. Besides its importance in medicine, people buy and sell the common plantanna as a pet. Some local people in Africa also once collected the frogs for food.

Conservation status: Because these frogs live in many areas and do well in a variety of different habitats, the species is not considered to be at risk. ∎

TROPICAL CLAWED FROG (Silurana tropicalis): SPECIES ACCOUNTS

Physical characteristics: If a frog were made of milk chocolate and started to melt, it would look something like the tropical clawed frog. This frog has a light chocolate brown or slightly greenish brown, flat, round blob of a body. Tiny gray and black marks fleck its back, and a row of 18 to 20 "stitches" runs down each side of the body from the eye to the rump. Its underside is white or light gray with some black blotches. It has a small, flat, round head with two beady eyes on top, and a tiny tentacle hanging below each eye. Its hind legs are large and pudgy. When the frog is sitting, its back legs and its much smaller forelegs stick out from the side of the animal, rather than tucking against the body as is common in most other frog species. Three claws are visible on its feet. The females are slightly larger than the males and usually grow to 1.7 inches (4.3 centimeters) long from snout to rump. The males typically reach 1.4 inches (3.6 centimeters) long.

Geographic range: Tropical clawed frogs live in western Africa.

Habitat: The tropical clawed frog lives mainly in water within tropical forests, but sometimes it lives in the ponds of grassland areas that are alongside forests. It does not exist in mountain areas.

Diet: From what little is known about its diet, scientists believe the tropical clawed frog eats almost anything it can find, including various invertebrates and tadpoles. They are not sure if it eats its own tadpoles or those of other frogs.

Behavior and reproduction: This frog stays in the water most of the time, but it will move about on land from pond to pond on very rainy nights. They behave differently during the dry season, when water can be scarce. Those that live near rivers hide in holes or under stone and roots during the daytime and sit at night in small, rocky pools of water left standing along the river. Those that live in ponds that lose much of their water in the dry season will bury themselves in the muddy pond bottom.

The tropical clawed frog takes advantage of the year-round warmth of the climate where it lives and may mate whenever a heavy rain drenches the land. The males will make their rattling call at night from large forest ponds or small forest pools. When a male finds a female, he climbs onto her back and holds on above her hind legs to mate with her. She lays her eggs in the water, and the eggs stick to underwater plants.

Tropical clawed frogs and people: People rarely see this frog in the wild, and it is not popular in the pet trade.

Conservation status: This species is not considered to be at risk. ∎

SURINAM TOAD (Pipa pipa): SPECIES ACCOUNTS

Physical characteristics: The Surinam toad is nothing less than bizarre. Its body is so flat that it appears as if it has been run over. It has a triangular-shaped head that comes to a point at the end of the snout. Its fairly short hind legs have huge, webbed feet. Each of the toes on its short front legs is split at the end into four pieces that almost look like four more small toes, and each of these small "toes" is split again at the tip into two more. It also has tiny, spiny bits of skin poking out from the sides of its mouth. Two tiny eyes may peer out from the top of the head, but sometimes they are completely hidden underneath flesh. The Surinam toad also has two slits for nostrils on the top of its head. Its body is dark brown, grayish brown, or tan and often has a blotched pattern. Its underside has a T-shaped marking with the top of the T running across the chest. Females are usually a bit larger and can grow to 4 to 7 inches (10.5 to 17.1 centimeters) long from the snout to the rump. The males usually reach no more than 6 inches (15.4 centimeters) long.

Geographic range: The Surinam toad lives in the northern and central parts of South America, including Colombia, Venezuela, Guyana, French Guiana, and Surinam (also sometimes spelled Suriname) in the north, and south into Brazil, Ecuador, Peru, and Bolivia. It also lives on the West Indies island of Trinidad.

Habitat: The Surinam toad lives on the bottoms of mucky ponds and swamps, as well as in slow-flowing streams and rivers throughout the lowland rainforests. It does not live in the mountains.

Diet: This toothless, tongueless frog catches small fish and invertebrates underwater by lunging at the prey while opening wide its mouth and blowing up its body. This sucks in the water and the prey together. Using its front feet, it pushes the prey farther into its mouth.

Behavior and reproduction: This frog stays still and rests in the mucky water bottom much of the time, but it will move on land from pond to pond on very rainy nights. Because it usually stays out of sight, scientists know little else about its non-mating behavior. During the mating season, they call with a sharp clicking noise. The males grab onto the females in piggyback fashion, hanging on in front of her hind legs. The frog pair rolls over while floating in the water, and the female lays three to five eggs while she is in the upside down position. The eggs catch on the male's belly, then drop onto the female's back as the pair completes the roll. Instead of the eggs sticking to vegetation or floating off into the water as they do with most frogs, the eggs stay on the mother's back, where they become caught. Her skin swells up around the sides of each egg. In all, she may have about 50 eggs on her back, which remain there for the next three or four months. At that time, the eggs hatch right into froglets, which pop right out of her back.

Surinam toads and people: Some people eat these frogs.

Conservation status: This species is not considered to be at risk. ∎

FOR MORE INFORMATION

Books:

Channing, Alan. Amphibians of Central and Southern Africa. Ithaca, NY: Comstock Publishing Associates, Cornell University Press, 2001.

Halliday, Tim, and Kraig Adler, eds. The Encyclopedia of Reptiles and Amphibians (Smithsonian Handbooks). New York: Facts on File, 1991.

Maruska, Edward J. Amphibians: Creatures of the Land and Water. New York: Franklin Watts, 1994.

Miller, Sara Swan. Frogs and Toads: The Leggy Leapers. New York: Franklin Watts, 2000.

Rödel, Mark-Oliver. Herpetofauna of West Africa. Vol. 1, Amphibians of the West African Savanna. Frankfurt: Edition Chimaira, 2000.

Tinsley, R. C., and H. R. Kobel, eds. The Biology of Xenopus. London: Clarendon Press, 1996.

Web sites:

"African Clawed Frog." Smithsonian National Zoological Park.http://nationalzoo.si.edu/Animals/ReptilesAmphibians/Facts/FactSheets/Africanclawedfrog.cfm (accessed on February 14, 2005).

"Frog Facts: About the African Clawed Frog." Fluffy's Frog Pond.http://fluffyfrog.com/FrogPondFactsF.html (accessed on February 14, 2005).

"Froggiecam." Ferrell Lab, Stanford University.http://www.stanford.edu/group/ferrelllab/frogtank.html (accessed on February 12, 2005).

"Frogs: A Chorus of Colors." American Museum of Natural History.http://www.amnh.org/exhibitions/frogs/featured/laboratory.php (accessed on February 14, 2005).

"Xenopus laevis African Clawed Frog." Western Ecological Research Center, San Diego Field Station, U.S. Geological Survey.http://www.werc.usgs.gov/fieldguide/xela.htm (accessed on February 12, 2005).