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Corroboree Frogs – the rarest of them all

With distinct yellow and black markings, the Corroboree frog (Pseudophryne corroboree) is one of the most iconic, frog species in Australia. They occur as two species (the Northern Corroboree Frog and the Southern Corroboree Frog) and are only found in Australia’s alpine region – around Mt Kosciuszko National Park. When fully grown, the Corroboree frogs are about 3cm in length and have a lifespan of between 8 – 10 years. Unlike most other species of frogs, the Corroboree frogs walk and are thought to be the only species to produce their own toxins. This means that these little frogs do not have any natural predators.

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Corroboree frog walking on moss. Image reproduced from Jean-Paul Farrero.

The Corroboree frog also has the distinction of being one of the rarest frogs in the world. At one point, its population in the wild was thought to be less than a hundred. In 2013, no breeding was recorded by conservationists. Concern for their survival has led to the Corroboree frogs being listed as ‘critically endangered’ by the IUCN and under the EPBC Act (1999). The frogs are also listed as ‘endangered’ under the NSW Threatened Species Act (1995) and the ACT Nature Conservation Act (2014).

The survival of Corroboree frogs in the wild is undermined by a number of interrelated factors. The first is the natural breeding cycle. It takes the frogs between 3 – 4 years to reach sexual maturity. Mating occurs over the summer months when the females lay about 40 eggs in the alpine bogs and ponds. By frog standards, 40 eggs is considered a very small clutch size. Once the eggs are laid, they develop into tadpoles but do not hatch until the rainy autumn months.

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Alpine habitat at Mount Kosciuszko National Park where the Corroboree frogs live. Image reproduced from Jean-Paul Ferrero.

The reliance on the rainy season makes the frogs vulnerable to the impacts of climate change. The altered rainfall patterns will interfere with the hatching of Corroboree tadpoles, making it harder to maintain a healthy, functioning population. Further, the warmer, hotter global temperatures means that alpine areas will continue to shrink, restricting the frog’s habitat range.

Chytrid – the invisible killer

Breeding and climate change aside, Corroboree frogs are threatened by something more sinister and insidious. An invisible predator that has slowly spread itself across Europe, Africa, America and Asia, and decimated many species of frogs. Not much is known about the origins of the chytrid fungus (Bactrochochytrium dendobatidis). There is consensus that globalisation has made the spread of the fungus from one country to another easier. Once introduced into an uninfected environment, the chytrid spreads in one of two ways. Firstly, through direct contact between infected individuals. Secondly, the fungal spores themselves spread through water.

The fungus attaches itself to keratin, the protein component of skin and interferes with osmosis – the frog’s ability to breathe through its skin. Since it first emerged in 1993, the fungus has gone onto infect ten species of Australian frogs. While some species such as the Corroboree frog titter on the brink, others have succumb to the chytrid fungus. The current extinction list includes; the Southern and Northern Gastric-brooding frog, the Sharp-snouted day frog and the Southern day frog.

There are also some species such as the Common Eastern Froglet (Crinia signifiera) and the Alpine Tree Frog (Litoria verreauxii alpina) which are thought to act as reservoirs for the chytrid fungus. The reservoirs provide a safe hiding space for the chytrid, while also helping transport the fungus to new locations.  

Both reservoir species, have habitat ranges that overlap with the Corroboree frogs. This means that preventing the contamination of healthy populations and containing the spread of the chytrid difficult. The eradication of the fungus from the environment is made difficult by its affinity to cold, wet environments. This makes the alpine region of Mt Kosciuszko National Park a stronghold for the invisible killers and the Corroboree frogs easy targets.

Conservation response – the silver lining

Even though the plight of the Corroboree frog seems dire, there are some reasons for optimism. Firstly, the chytrid’s strength – its ability to bind to the protein keratin also happens to be a weakness. This is because during their egg and tadpole stage, frogs lack keratin. In other words, they are infection free.

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Corroboree frogs eggs. Image reproduced from Lydia Fucsko (2011).

Over the past decade, a dedicated, captive breeding program between Taronga Zoo, Melbourne Zoo, Healesville Sanctuary, Tidbinbilla Nature Reserve and the Amphibian Research Centre has been capitalising on this.

Infection free enclosure. Image credit: McFadden (2014)
Infection free enclosure where Corroboree frogs are kept in the wild. Image credit: McFadden (2014)

The primary aim of the program is to create an infection free, insurance population. At Taronga Zoo, the carefully regulated room used for keeping and breeding the Corroboree frogs also serves as an exhibition and education space. Thus far, the program appears to be successful. The eggs and tadpoles collected from the captive frogs are being introduced into the wild. To enhance their survival, they are released in remote, alpine areas that are infection free. These efforts have seen numbers slowly climb from a few hundred to about two thousand. Having an insurance population also gives conservationists hope. A hope that it buys valuable research time in which some sort of solution(s) to completely halting or eradicating the chytrid fungus at the molecular level.

The Corroboree frog, with its distinct yellow and black markings is an iconic Australian species. One that has been pushed to the brink of extinction by a silent killer that has slowly spread across the globe. Some species of frogs have been entirely decimated, while others are slowly being lost. A few years ago, the Corroboree frogs were thought to be one of the rarest of species in the world. However, a dedicated and long-term captive breeding program in Australia has helped to create an insurance population of Corroboree frogs that are infection free. Its success is helping boost numbers in the wild as well as buying valuable time so that a solution to the fungus can be found.