Darwin’s frogs are aptly named, because they provide a wonderful example of evolutionary adaptation, blending in into their habitat of the leaf litter, with their body plan resembling a leaf. Unfortunately one of the two species, the northern Darwin’s frog (Rhinoderma rufum) hasn’t been seen since 1980 and a recent survey had no luck in finding any individuals, although its vast distribution in the past gives some reason for hope. The southern Darwin’s frog (Rhinoderma darwinii) has declined severely in numbers and its distribution throughout its range is now patchy with small population sizes. A study that appeared in PLOS One last week has shed some light on the dwindling numbers of these enigmatic animals. These species are both presumed to be yet another victim of the chytrid fungus that has been held responsible for decimating amphibian populations worldwide. A team of researchers led by Claudio Soto-Azat of the Universidad Andres Bello in Santiago and the Zoological Society of London (ZSL) found this out by conducting an analysis on dead and living specimens of the Darwin’s frogs and on other anuran (frog and toad) species living in their vicinity that might be carriers of the disease. Their results showed a high prevalence of the chytrid fungus in areas where the Darwin’s frogs have undergone dramatic declines.
Southern Darwin’s frog (Rhinoderma darwinii)
Credit: Claudio Soto-Azat
On his voyage with the Beagle, Darwin came across these beautiful frogs when he visited the area around Valdivia, Chile in 1835. Although they make a compelling example of adaptation to the environment, underpinning his theory of ‘descent with modification’, they are not named in honor of Darwin’s theory, but in honor of his naturalistic work, being the first to ever see them. Their range is not restricted to the area where they were discovered: the northern Darwin’s frog, is endemic to central Chile, while the southern Darwin’s frog is found in south and southern Chile and in adjacent areas of Argentina. Another feature, besides their leaflike appearance, that makes them such a wonder of evolution is their mode of reproduction: the males of these species rear their offspring in their vocal sacs. In R. rufum the young tadpoles spend two weeks of their development in the vocal sacs of their father and are then released into a stream to develop further, but in R. darwinii the entire transformation from egg to froglet takes place in the vocal pouch of the male.
The reasons for the decline and maybe even extinction of the northern Darwin frog and the decline of its southern counterpart remain poorly understood. Habitat degradation seems to play an important role, big parts of the range of the Darwin’s frogs have been affected by the replacement of native forests with plantations of pine and eucalyptus and because of land use change to agriculture. However, populations of the species that are situated in national parks or in particular areas where no such human pressure exists, are not doing well either, so this can’t be the entire story. Therefore it has been thought that chytridiomycosis, an infection disease caused by the chytrid fungus, could be responsible for casualties as well. This disease has been associated with extinctions and population declines in amphibians worldwide. To get a feeling of how bad this disease has been for amphibians, read this quote about chytridiomycosis, describing it as “ the worst infectious disease ever recorded among vertebrates in terms of number of species impacted and its propensity to drive them to extinction” .
To identify whether the chytrid fungus indeed can be held responsible for the deteriorating populations, the researchers sampled live individuals of the species (without harming them of course) and looked for the presence of the fungus in other frog and toad species that live in areas where the Darwin’s frogs experience dwindling numbers or have become extinct. This added up to almost four years of fieldwork in Chile and Argentina . In the archives of zoological museums, individuals of the species, collected in past times, have been preserved and these dead frogs were sampled as well, to see whether the chytrid fungus had been present in the past and could account for the disappearance of the northern Darwin’s frog some decades ago.
Male southern Darwin’s frog with froglet, just emerged from its vocal sac
First of all, no individuals of the northern Darwin’s frog were found during the fieldwork, adding to the likelihood of its extinction. Evidence from the research points to chytridiomycosis as the culprit, as traces of the fungus have been found in an archived individual of this species that belonged to the largest known population of R. rufum in Chile. Moreover, the results from the archived specimens indicate that the chytrid fungus entered the area around 1970; this coincides with the timing that has been proposed for the first amphibian deaths caused by the disease, and would have been well in time to affect the Darwin’s frogs.
Luckily, enough individuals of the southern Darwin frog could be captured and sampled to make an analysis of the impact that the chytrid fungus might have. Among these frogs, infected individuals have been found, although the infection rate of this frog species was lower than the infection rate of the other frogs and toads living in the same area. This might be due to the fact that development of the tadpoles in this species takes place entirely within the vocal sac of the male and because the chytrid fungus is spread through the water, they might come in contact with the fungus less frequently than other species. This might also provide an explanation why the northern Darwin frog instead of the southern Darwin’s frog has presumably gone extinct: tadpoles in the former species, after residing for two weeks in the vocal sac of their father, still need to spend roughly 120 days in the water to complete their metamorphosis, making them more vulnerable to infection by the fungus.
However, the low observed infection rate of the southern Darwin’s frog might also mean that this species copes very badly with the disease and dies soon after infection, hence finding alive individuals carrying the disease would be rare. Whatever the reason may be, another result strongly indicates that the species doesn’t tolerate the chytrid fungus in its surroundings: at sites where the frogs have become extinct or have declined severely, the prevalence of the chytrid fungus in other anuran species, was significantly higher than at sites where relatively stable populations of the species occur. So wherever the chytrid fungus was abundant, Darwin’s frogs were not.
So now it has become clear that chytridiomycosis very likely has a negative effect on populations of these magnificent creatures. Still the question remains how severely individuals are affected by the disease and whether some of them might be relatively resistant to it. It seems for now safest to assume that the further spread of the disease will have a large impact on these evolutionary unique frogs. And so, with declining numbers of the species, conservation efforts are dearly needed and should also focus, besides securing their habitat, on containing the chytrid fungus.
