Darwin's frog (Rhinoderma darwinii ), also called the southern Darwin's frog, is a rhinodermatid frog native to the forest streams of Chile and Argentina. It was first described by the French zoologist André Marie Constant Duméril and his assistant Gabriel Bibron in 1841, and is named after Charles Darwin, who had previously discovered it in Chile during his voyage on HMS Beagle. They are carnivores.
The most striking feature of this frog is the tadpoles' development inside the vocal sac of the male.
Darwin's frog is a small species with a snout–to–vent length of 2.2 to 3.1 cm (0.9 to 1.2 in). The snout is elongated into a fleshy proboscis which gives the head a triangular shape. The limbs are relatively long and slender. The front feet are not webbed, but some of the toes on the back feet usually are.
The upperparts of this species are variably colored in brown or green. Some brown individuals may have weakly defined V-shaped markings on the back, have central upperparts that are distinctly lighter brown than the flanks, or have whitish front limbs. Females are typically brown and tend to stay on substrates that match this color. Males are far more variable and occur on a wider range of substrates; especially brooding males often are partially or completely green above. The throat is brownish and the remaining underparts are black with large white blotches in a pattern that is unique to each individual.
Darwin's frog is found in Chile and Argentina. In Chile, its range extends from Concepción Province to Palena Province and in Argentina from Neuquén Province and Río Negro Province. It is found in glades and forested areas at altitudes of up to about 1,100 m (3,600 ft) above mean sea level. It is also found in bogs and near slow-moving streams. It is found in a variety of vegetation types and it appears that a mixture of grassland, mossy areas, coarse woody debris, and young trees and bushes in a mature native forest provides its optimum habitat requirements. Short vegetation increases the retention of water while decreasing the temperature of the soil and provides concealment from predators. The population is fragmented and this frog has poor dispersal ability.
Darwin's frog feeds on insects and other arthropods. It not only has to hunt, but also needs to hide from its predators. It relies on camouflage to avoid predators, lying on the ground looking like a dead leaf until the predator passes by. Another defensive position is turning on the back and exposing the boldly patterned ventral surface.
The female Darwin's frog lays up to forty eggs among the leaf litter. The male guards them for about three to four weeks until the developing embryos begin to move, and then he ingests the eggs and holds them in his vocal sac. They hatch about three days later and he continues to carry the tadpoles around in his vocal sac where they feed off their egg yolks and secretions produced by the wall of the sac until metamorphosis. At this stage, about 6 weeks after being swallowed, while previous authors believed that during this period the adult male eats nothing, G B Howes, in 1888, dissected a brooding male and found its stomach full of beetles and flies and its large intestine clogged with "excreta like that of a normal individual." He concluded "that this extraordinary paternal instinct does not lead up to that self-abnegation". Following this, the small frogs hop out of the male's mouth and disperse.
Darwin's frog has undergone significant population declines due to habitat loss and degradation, largely from conversion of native forests to tree plantations. The amphibian disease chytridiomycosis, caused by Batrachochytrium dendrobatidis fungal infection, is also a probable factor, "particularly from the northern part of their historical range". Populations on Chiloé Island were found to be considerably more abundant than on the mainland.
Since 2018, the species is classified as Endangered on the IUCN Red List. A 2013 study reported results of a population survey conducted from 2008–2012, which found the species at just 36 of 223 previously recorded habitat sites, with small populations at those sites. The recent change in its conservation category in IUCN from Vulnerable to Endangered, arose from the Chile's amphibian reevaluation workshop for the Red List (Soto-Azat et al., 2015). The justification for its current category is due to its limited occupation area (estimated at 264 km2), severe fragmentation of its populations and continued decline.
Since October 2021, R. darwinii has been classified as Critically Depleted by the IUCN. It was determined that the survival of the species is highly dependent on conservation activities and its recovery potential is high.
Due to its decline in the wild, captive colonies have been established as a precaution at two zoos in Chile, the National Zoo (working with the US Atlanta Botanical Garden) and Concepción Zoo (working with the University of Concepción and Germany's Leipzig Zoo).
