Darwin's frog
Rhinoderma darwinii
TAXONOMY
Rhinoderma darwinii Duméril and Bibron, 1841, Valdivia, Chile. No subspecies are recognized.
OTHER COMMON NAMES
French: Le rhinoderme de Darwin; German: Darwin-Nasenfroschs; Spanish: Ranita de Darwin, sapito de Darwin, sapito vaquero.
PHYSICAL CHARACTERISTICS
These are small frogs; males are up to 0.9–1.1 in (22–28 mm), females are 1–1.2 in (25–31 mm), with moderately developed membranes between the first and second toes and between the second and third toes. The membrane between the third and fourth toes is smaller, and there is no membrane between the fourth and fifth toes. The metatarsal tubercle is evident, but less prominent than in R. rufum.
DISTRIBUTION
This species occurs in central and southern Chile, from the province of Maule south to the province of Aisén, from 0–4,921 ft (0–1,500 m) elevation. In Argentina, the frogs occur near the border with Chile, in the provinces of Neuquén and Río Negro.
HABITAT
The frogs are found both in primary and in disturbed forest. They are also commonly found in open areas around human habitation, and in open wooded or grassy areas. Most individuals are found in or near swampy areas or slowly running water.
BEHAVIOR
This species is primarily diurnal, but males also call at night. Some individuals display an unusual behavior when disturbed. They flip over onto their backs, revealing their contrasting black and white undersides. If a frog near a stream is frightened, it may jump into the water and float downstream on its back.
FEEDING ECOLOGY AND DIET
Darwin's frogs are sit-and-wait predators. By day, they sit in one place and snap up moving insects and other small invertebrates that come within striking distance.
REPRODUCTIVE BIOLOGY
The male mating call is a rapidly repeated "piiiip, piiiiip, piiiiip, piiiiip." Calling is most prevalent beginning in the spring
and continuing through the breeding season (November through March).
Observations made in captivity reveal that a male leads a female to a sheltered place that serves as the site for egg deposition. After considerable courtship movements by both frogs, the female crawls underneath the male. He holds onto her very loosely, in contrast to the typical strong amplectant hold of most frogs.
Darwin's frogs deposit and fertilize large eggs (about 0.16 in/4 mm in diameter) on land. In a population studied from the far south of the range, clutch size was estimated to be three to seven eggs. The male stays near the eggs for about 20 days, until the eggs are nearly ready to hatch. At that point, the male takes the eggs into his mouth where they enter his vocal sac and soon hatch. The tadpoles develop within the vocal sac for the next 50–70 days. After the young metamorphose, they crawl back into the father's mouth. The father opens his mouth and the froglets hop out onto land.
The tadpoles lack the typical morphology of free-swimming tadpoles. They do not have external gills, spiracle, beak, or keratinized teeth, and their caudal fins are poorly developed.
Studies of the lining of the vocal sacs of brooding males suggest that the epithelial cells secrete a substance that is taken up by the tadpoles through their skin. Tracers experimentally introduced into the lymphatic sacs moved into tissues of the tadpoles, further supporting the idea that tadpoles receive nutrients from the lining of the vocal sac.
CONSERVATION STATUS
Listed as Data Deficient by IUCN but Vulnerable by CITES. Although the frogs are locally common in some areas (particularly at low elevations), populations are declining or disappearing in other areas (especially at high elevations). The causes of these declines and disappearances are unknown, but habitat destruction is a major threat. Some areas that previously supported dense populations of Darwin's frogs are now planted in non-native pine or eucalyptus, or have been converted to pastures or human residential areas. Climatic change may also be affecting the species, as the climate throughout much of the range is warmer and drier than it was 15–20 years ago. The frogs may also be affected by increased levels of ultraviolet radiation, as the frogs are diurnal and often bask in sunlight.
SIGNIFICANCE TO HUMANS
None known.
