Vocal Sac-Brooding Frogs: Rhinodermatidae

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 The male Darwin's frog stays with his eggs until they are almost ready to hatch, then scoops them up with his mouth. The eggs slide back into his vocal sac. Since he no longer needs to use the vocal sac to call in females, it provides a safe spot for the eggs to hatch into tadpoles. (Illustration by Wendy Baker. Reproduced by permission.) 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. ∎