Why Darwin’s Mouth-Brooding Frogs are Vanishing

Why Darwin's mouth-brooding frogs are vanishing

The population of Darwin’s mouth-brooding frogs appear to be mysteriously vanishing and, according to a new research study, a deadly fungal disease could be responsible for the mass extinction event.

What Are Darwin’s Frogs?

The frogs in question belong to the Rhinodermatidae family, comprising of the Northern Darwin’s frog (Rhinoderma rufum) and the Southern Darwin’s frog (Rhinoderma darwinii), both of which dwell within temperate forests, rivers and swamps around central and south Chile.

The Southern Darwin’s frog was originally discovered by Charles Darwin on the island of Lemuy of the Chiloé Archipelago, which lies just off the coast of Chile. It was later described by French Zoologist André Marie Constant Duméril.

R. darwinii is a frog with rather unique aesthetics, exhibiting a predominately brown or green tinge and a singular, white streak running down it back. The cunning creature uses its color and appearance as camouflage and plays dead around perusing predators, looking remarkably like a leaf when static. The petite amphibian’s front feet are not webbed and its main body measures a length of around two to three and a half centimeters.

Meanwhile, the Northern Darwin’s frog was later described in 1902, but was not defined as a distinct species until 1975.

The Mouth-Brooders

One of the most unusual aspects of these frogs is their mouth-brooding capabilities. The female frogs will lay around 30 or so eggs, for which their male counterparts are then tasked with guarding. Once the eggs hatch, the male scoops up the offspring that have managed to survive and carries them around in its vocal pouch.

The tadpoles rapidly develop inside the saggy skin of the male’s chin – feeding off their egg yolk – before hopping away, once they are large enough to swim of their own accord.

Rhinodermatidae a Dwindling Family

R. rufum has not been observed since 1980, whereas R. darwinii appears to be rapidly disappearing from a number of regions, where it was once considered endemic. R. rufum is currently on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species and ranks in at number 45 on the amphibian evolutionary distinct and globally endangered (EDGE) list. Meanwhile, the IUCN defines R. darwinii as “vulnerable,” but there is little statistical data on their abundances or distributions.

The reason for the decline in Darwin’s unique, mouth-brooding frogs remains poorly understood, to this day. Some researchers have conjectured that the population slump may be attributed to habitat degradation, principally caused by mass substitution of native forest for pine and eucalypt plantations.

However, these Southern and Northern Darwin’s frogs have been disappearing from wild protected areas (WPAs) of National Parks, as well as a number of other untouched and flourishing ecosystems – a finding that has confused researchers for some time.

Since Darwin’s frogs are diurnal, the research team conducted prior surveys during the day to maximize the likelihood of encountering the elusive amphibians. The investigations were carried out between October and March, at a time when the frogs are known to be most reproductively active.

Ultimately, the group concluded that the species R. rufum was likely extinct, whereas R. darwinii was still in existence across multiple sites, but at a much lower abundance than originally anticipated.

The Deadly Bd Fungus

The group looked at habitat destruction and deforestation to determine whether these factors contributed significantly to the decline of Darwin’s frog numbers. However, lead author Andrew Cunningham and fellow researchers from the Zoological Society of London (ZSL), explored the contingency that these frogs had come under attack from a fungal parasite.

Batrachochytrium dendrobatidis prevalence in frogs of Chile
Map showing the prevalence of Batrachochytrium dendrobatidis fungus in Chile, between 2008 and 2012. The size of the circles corresponds to the population size studied. The circles are also color-coded to show infected (red) and non-infected (blue) frog populations.

Many amphibian populations have been struck down by chytridiomycosis, caused by the water-borne pathogen Batrachochytrium dendrobatidis (Bd), with the first outbreak seen spreading through the frog population of Queensland, in 1993.

The fungal infection manifests as a number of physical symptoms, including convulsions, accumulation of sloughed skin, reddening of the frog’s skin and occasional hemorrhage and ulceration. Infected frogs demonstrate radical behavioral changes and do not respond to predation, fail to seek protective shelter and display general apathy.

The group analyzed archived samples of Northern and Southern Darwin’s frog species, collected between 1970 and 1978. polymerase chain reaction techniques were employed to detect the presence of the Bd fungus in these frog specimens, with a total of five Northern and Southern Darwin’s frogs testing positive; this represents an infection prevalence of one percent of all tested frogs.

Meanwhile, swabs that were taken from extant frog populations, excluding the reportedly extinct R. rufum, indicates that around 12.5 percent of all specimens were infected with the killer fungus. The stark difference in the number of infected frogs collected over three decades ago, relative to those of today, suggests that the fungus could have spurred the species’ decline.

Claudio Soto-Azat, a Ph.D. candidate working at Andres Bello University in Santiago, Chile, offers his perspective as to how the group’s results should be interpreted:

“Amphibians have inhabited the earth for 365 million years, far longer than mammals. We may have already lost one species, the northern Darwin’s frog, but we cannot risk losing the other one… There is still time to protect this incredible species.”

The team claim to have found an inverse relationship between Bd prevalence and latitude, with the disease, seemingly, not having reached the southern climes of the country. On this basis, the researchers have suggested that biosecurity protocols be introduced to prevent dissemination of the fungus to other regions.

By James Fenner

PLoS One Journal1

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National Geographic

BBC News

IUCN Link1

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