The Northern Darwin’s frog is one of only two frogs in the world where the young undergo part of their development in the parent’s mouth. Eggs are laid on damp ground and, when the developing tadpoles start to wriggle in their egg capsules, the guarding male swallows them into his vocal sac. Here they stay until their jaws and digestive tracts are fully formed, where upon the male carried them to a stream. This species has not been seen since around 1980 and it could have been driven to extinction by a mystery disease, possibly the fungal disease chytridiomycosis (responsible for many amphibian declines globally), although this has not previously been reported from Chile.
The Cycloramphidae is a family of nearly a hundred species that diverged from all other amphibian lineages some 65 million years ago, around the time of the extinction of the dinosaurs. In terms of mammalian evolutionary comparisons, they are as dissimilar from their closest relatives as whales are from giraffes.
Darwin’s frogs – 2 small species in the genus Rhinoderma – formally constituted their own separate family, the Rhinodermatidae. The genus Rhinoderma is only known from the temperate forests of southern Chile and Argentina, and there is some overlap in range of the two species. Their classification as a distinct family was chiefly on the basis of their highly unusual mouth-brooding behaviour. Both species of Darwin’s frog undertake partial care of the offspring in their mouths or, more specifically, their vocal sacs. The males of these two species guard the eggs until hatching and then swallow the tadpoles into their vocal sac. In the case of the Northern Darwin’s frog, the tadpoles are then transported from the damp nest site to pools of water where they are released. However, in the slightly less endangered relative, named the Southern Darwin’s frog, the tadpoles develop into froglets within the vocal sac, hopping out of the mouth about 50 days later. The male’s vocal sac is particularly large and extends half way down his belly.
The Northern Darwin’s frog is a small species, measuring 31 mm in total length for males and 33 mm for females. A distinguishing feature of the Darwin’s frogs is the presence of a fleshy “proboscis” or narrow pointed projection of the tip of the snout which resembles a tiny Pinocchio’s nose. The forelimbs and hindlimbs are slender, and the toes are extensively webbed. This species can be highly variable in colour. The back or dorsal surface may be uniformly tan, brown, reddish brown, pale green, dark green, or a mixture of brown and green. The ventral (or lower) surface has black and white splotches. Brooding males show enlarged vocal sacs.
Both species in the genus Rhinoderma (the Northern Darwin’s frog, R. rufum and Southern Darwin’s frog, R. darwinii) exhibit a highly unusual form of parental care. They are commonly referred to Darwin’s frogs or the “mouth-brooding“ frogs. This second epithet is derived from the fact that in both species, the tadpoles spend part of their development in the vocal sac of their father. Darwin’s frog produces non-feeding tadpoles which, upon hatching in a damp nest on the ground, are swallowed by the male and may spend around 50 days metamorphosing in his vocal sac. They may also receive some nutrients from the males during this time, and hop out of the male’s mouth as fully formed froglets. In the Chile Darwin’s frog, however, the tadpoles stay in the male’s mouth is not so protracted, and they merely hitch a ride in their father’s vocal sacs to a pool of water, where they continue their development as free-living tadpoles. In the Northern Darwin’s frogs, the tadpoles only undergo the first stages of their development in the male’s mouth.
During the mating season, male Northern Darwin’s frogs start producing a mating call consisting of a rapid “pip, pip, pip, pip...“ with long pauses between repetitions. Calls are made while the males are hidden in vegetation, on the surface of moist soil. The dominant frequency of the call is about 3000 Hz, which is at the upper limit of the range for frogs found in this species’ temperate forest habitat. The call of the Northern Darwin’s frog is very similar to that of the Darwin’s frog, which is a rare finding for frogs that are both related and sympatric (co-occurring). It is not known how tadpole transport inside the vocal sacs might affect male calling characteristics.
The female lays a clutch of 12-24 small eggs on moist ground in leaf litter, which are fertilised and then guarded by the male. They start to move after about eight days later, when the developing tadpoles first wriggle within the eggs. This prompts the males Northern Darwin’s frog to take up the eggs into his vocal sac. The eggs subsequently hatch there and the tadpoles remain in the male frog’s vocal sacs. They are incubated in the males mouth to the point of developing fully-formed keratinized jaws and a functioning digestive system, where upon the male releases the tadpoles into a pool of water or stream. The tadpoles complete their development and undergo metamorphosis in the water.
The Northern Darwin’s frog is presumed to be a sit-and-wait predator which preys on small insects and other small invertebrates. This frog is terrestrial, and primarily diurnal or day-active.
This species has been recorded in leaf litter in temperate mixed forests, and also in bogs surrounded by forests. Northern Darwin’s frogs have been found near slowly running streams in wet temperate southern beech (Nothofagus) forest at elevations from 0 to 500 metres above sea level.
The Northern Darwin’s frog formerly occurred in small, isolated populations, and was fairly regularly seen until around 1978. Since around 1980 there have been no confirmed reports despite several attempts to relocate the species. This species may already be extinct.
The Northern Darwin’s frog is listed as Critically Endangered in the IUCN Red List of Threatened Species because of an observed population decline, estimated to be more than 80% over the last ten years, based on the apparent disappearance of most or all of the population for reasons that are not understood.
The Northern Darwin’s frogs are not known from any protected areas and there are currently no conservation measures ongoing for this species.
This project is investigating the enigmatic declines of the Darwin's frogs in Chile.
The species may already be extinct, but further survey work is urgently required to determine whether this is indeed the case. Concurrent with new field surveys, comprehensive disease monitoring of the habitat should take place to find out if the range of the Northern Darwin’s frog contains any amphibian pathogens, such as Batrachochytrium dendrobatidis, the chytrid fungus.
In view of the possible risk of disease, any surviving individuals may need to form the basis of an ex situ captive breeding programme.
Claudio is investigating the causes behind the enigmatic declines of the Rhinoderma Darwins frogs.
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