Vocal Sac-Brooding Frogs: Rhinodermatidae
VOCAL SAC-BROODING FROGS: RhinodermatidaeDARWIN'S FROG (Rhinoderma darwinii): SPECIES ACCOUNT
The family of vocal sac-brooding frogs has only two species: Darwin's frog and Chile Darwin's frog. Both of these little frogs have flattened heads that come to a fleshy point at the tip of the snout. With their large eyes and thin, nearly beak-shaped snout, these frogs almost have a bird-like face. The pointy snout tip is especially noticeable because it comes at the end of two ridges, one found on each side of the body and extending from nearly the rump all the way to the snout. Another fainter ridge runs along the upper lip to the bottom of each large, gold-colored eye and down each front leg. The front legs are shorter than the hind legs, but all four are slender. The toes on the front feet have no webbing, but most if not all of the hind toes are at least partially webbed. Both species have a bump, or tubercle (TOO-ber-kul), on each hind foot. Their bodies may be light brown, reddish brown, brown, light green, or dark green, sometimes with brown, gray, or green markings. At least part of the underside, usually including the hind legs and lower part of the belly, is black with large white spots or blotches.
Although the two species look much alike, they are slightly different. The Chile Darwin's frog has webbing between all of the toes on its hind feet, while the Darwin's frog does not have any webbing between its outer two hind toes. In addition, the tubercles on the hind feet of the Chile Darwin's frog are larger than those on the Darwin's frog.
The two members of this family are small frogs. A male Darwin's frog grows to 0.9 to 1.1 inches (2.2 to 2.8 centimeters), and the female reaches 1 to 1.2 inches (2.5 to 3.1 centimeters). The Chile Darwin's frog is slightly larger. The male grows to 1.2 inches (3.1 centimeters), and the female reaches 1.3 inches (3.3 centimeters) in length. The males and females of both species look much alike except during the breeding season when the male's chest may be puffed up with eggs or tadpoles.
As its name suggests, Chile Darwin's frog lives in Chile. In particular, its home is in the central part of the country in areas between about 160 and 1,640 feet (50 to 500 meters) above sea level. Darwin's frog lives in the same area as Chile Darwin's frog and also farther south through Chile and into far western Argentina. They can be found from sea level to about 4,900 feet (1,500 meters) above sea level.
These frogs live in wet forests where beech trees grow and sometimes in open fields near houses and buildings. They usually do not wander too far from either wetlands or lazy streams.
Scientists do not know for sure, but they think these two species eat whatever insects and other invertebrates they can find.
DIFFERENT CALL, DIFFERENT SPECIES
Until scientists took a closer look, they thought Darwin's frog was the only member of the vocal sac-brooding family. One of the clues they used to decide that the frogs were actually two different species instead of one was that some males were making one type of call and others were making a different-sounding call. The call is very important among frogs because females use it to find males who are ready to mate. If a male's call is different—even if he looks just like every other male frog in the forest—a female will not answer his call and will not mate with him. An important requirement for animals to be of the same species is that the males and females must recognize each other as possible mating partners.
BEHAVIOR AND REPRODUCTION
Mostly active during the day, the vocal sac-brooding frogs likely find a spot in the forest or field and settle in to wait for an insect or other small invertebrate to wander by. The frogs then quickly nab the passing meal. The two species are active most of the year, but disappear in cold, winter months. The frogs probably wait for warmer weather from a sheltered spot under a layer of moss or a rotting log, but scientists do not know for sure where the frogs go in the winter. When the Darwin's frog is threatened, it flips onto its back and lies still. This display shows off the frog's bright black-and-white pattern, which may scare off a predator. The Chile Darwin's frog, which also has the black-and-white underside, probably does the same thing.
Once spring comes, the frogs again appear in the woods and meadows. Each male performs his nighttime calls from land and draws in a female. Darwin's frog calls quickly repeat a "pi-i-i-i-ip" over and over again. The male Chile Darwin's frog sings a fast "pip-pip-pip-pip" and waits a few seconds before repeating the short call. In both species, the female lays her eggs on the ground and leaves the parenting job to the male. A female Chile Darwin frog lays one or two dozen small eggs, while a female Darwin's frog lays three to seven larger eggs. The male in both species stays with the eggs until they are almost ready to hatch, then scoops them up with his mouth. The eggs slide back into his vocal sac, a balloon-like structure in the areas of his throat and chest that inflates and deflates when he calls. Since he has already mated and no longer needs to use the vocal sac to call in females, it provides a safe spot for the eggs to hatch into tadpoles. In about eight days, the male Chile Darwin's frog hops over to a stream or pool, and the young tadpoles squirm out of his vocal sac and into the water, where they later turn into froglets. In Darwin's frog, the tadpoles remain in the male's vocal sac for 50 to 70 days until they turn into froglets. Only then do they crawl outside.
VOCAL SAC-BROODING FROGS AND PEOPLE
People do not hunt and eat these frogs, and they do not commonly see them in the pet trade. Scientists are interested in the frogs because of the unusual role of the male in the development of his young.
According to the World Conservation Union (IUCN), Chile Darwin's frog is Critically Endangered, which means that it faces an extremely high risk of extinction in the wild; and Darwin's frog is Vulnerable, which means that it faces a high risk of extinction in the wild.
The Chile Darwin's frog is especially at risk. Since 1994, at least 80 percent of all members of this species have disappeared, and some scientists fear that it may already be extinct. People had seen Chile Darwin's frogs in their habitat until 1978, but trips to find the frog since then have found nothing. Scientists are not sure why this frog has vanished, but think that habitat loss, and particularly the removal of the plants where they live, may be part of the reason.
Physical characteristics: Darwin's frog, which is also sometimes called Darwin's toad, is a pudgy frog with a triangle-shaped head that ends in a very pointy snout. A ridge runs down each side of the body from the snout over the eye and almost to the rump. Its gold-colored eyes are on the sides of its head. No eardrums show. Its back, the top of its head, and the top of its legs are light brown with gray blotches. The underside of the frog is often light to dark brown on the throat and chest, and black with white blotches toward the belly and on the back legs. The toes on the front feet are unwebbed, but most of the toes on the hind feet have at least some webbing. The space between the two outer toes on the hind feet has no webbing. The frog also has a small bump, or tubercle, on its hind foot.
Females are slightly larger than males and can grow to 1 to 1.2 inches (2.5 to 3.1 centimeters) long from snout to rump. Males usually reach 0.9 to 1.1 inches (2.2 to 2.8 centimeters) in length. Besides their size, males and females look alike except during the mating season, when the male's chest may be puffed out because of its unusual breeding behavior.
Geographic range: Darwin's frog lives in central and southern Chile and continues across the border into far western Argentina.
Habitat: Darwin's frogs live in beech-tree forests and in fields, sometimes in areas near houses and buildings. They also live near and often in slow streams and swamps.
Diet: Ambush hunters, they sit still and wait for an insect or other small invertebrate to wander by closely enough to grab and eat it.
Behavior and reproduction: Darwin's frogs are active during the daytime, and they spend a considerable amount of time sunbathing, or basking. Although their body shape and color allow them to avoid notice much of the time, predators do sometimes discover this little frog. When the attacker approaches, the frog defends itself by throwing itself onto its back and playing dead. If the frog is near water, it jumps in first, then flips over and floats downstream while lying upside down. Both displays show off the frog's black and white underside and may frighten off a predator.
The most unusual behavior in this frog, however, is in its reproduction. Their breeding season begins in spring and continues into summer. In the daytime and occasionally at night during this time, each male makes his call, quickly repeating "pi-i-i-i-ip" over and over again. When he calls, he draws air into a vocal sac on his throat, which inflates and deflates like a balloon. When the female responds, he leads her to his nest, which is a hidden spot on land. She then squirms under his body, so that he winds up on top of her back. Instead of holding on to her back very tightly as other frogs do, the male Darwin's frog barely clasps her. She lays about three to seven eggs and leaves the male to take care of them for about 20 days when they are almost ready to hatch. He then gobbles them up. The eggs drop into his vocal sac, hatch there into tadpoles, and remain inside for another 50 to 70 days until they turn into froglets. During this time, the male's chest is puffed large with developing tadpoles. Scientists think the tadpoles survive by slowly eating the leftover yolk from their eggs, as well as some food provided by the male's body through the skin lining his vocal sac. The new froglets crawl out of the vocal sac, through their father's mouth, and to the outside, where they begin hopping about on land.
Darwin's frogs and people: Scientists are interested in this frog because of the unusual way that the male is involved in reproduction.
Conservation status: According to the IUCN, Darwin's frog is Vulnerable, which means that it faces a high risk of extinction in the wild. According to scientists who have studied the frog, it is now much less common than it was in the 1980s and 1990s, and it has vanished completely from some areas, including places inside national parks and other preserves. They believe a main reason for the frogs' disappearance in unprotected places is the loss of their habitat, especially due to logging of the forests where they live. In addition, the climate is becoming drier in this part of the world and may be making it harder for the frogs to survive. ∎
FOR MORE INFORMATION
Halliday, Tim, and Kraig Adler, eds. The Encyclopedia of Reptiles and Amphibians (Smithsonian Handbooks). New York: Facts on File, 1991.
Mattison, Chris. Frogs and Toads of the World. New York: Facts on File Publications, 1987.
Miller, Sara Swan. Frogs and Toads: The Leggy Leapers. New York: Franklin Watts, 2000.
Patent, Dorothy Hinshaw. Frogs, Toads, Salamanders, and How They Reproduce. New York: Holiday House, 1975.
Showler, Dave. Frogs and Toads: A Golden Guide. New York: St. Martin's Press, 2004.
Churchman, Deborah. "Hoppy Birthdays - Frogs." Ranger Rick.http://www.findarticles.com/p/articles/mi_m0EPG/is_4_36/ai_90445325 (accessed on March 10, 2005).
"Dads at Work." Time for Kids Classroom.http://www.timeforkids.com/TFK/class/ns/article/0,17585,490974,00.html (accessed on March 10, 2005).
"Strange Breeding: Darwin's Frog." Frogland.http://allaboutfrogs.org/weird/strange/darwins.html (accessed on March 10, 2005).