Giving frogs a leg up

With fewer than 100 southern corroboree frogs left in the wild, Emma McInerney believes that diet supplements can help give the species a fighting chance.

 
 Illustration by Leigh Douglas

Illustration by Leigh Douglas

 

Ethograms is a monthly column published in collaboration with the Australasian Society for the Study of Animal Behaviour (ASSAB), showcasing the work of early-career researchers. Amy LeBlanc is a master's student at the University of Melbourne, and an outreach officer for ASSAB.

High in the Australian sub-alpine region, a small frog is crawling through the wet moss. Its body is adorned with vibrant yellow and black stripes, warning any would-be predators to stay away. This is the corroboree frog, and it is one of Australia’s lesser known national treasures.

The frog’s name alludes to the species’ deep roots in Australian culture: “Corroboree” is derived from an Aboriginal word, referring to a gathering in which participants would often adorn themselves with yellow painted stripes. There are two different species of corroboree frog — the southern (Pseudophryne corroboree) and northern corroboree frog (P. pengilleyi) — both of which are endangered.

The corroboree frog is unique in many ways, making its drastic population decline a serious concern for conservationists. Research and breeding programs have been set up across the county to aid its survival. Emma McInerney, a PhD student at the University of Wollongong, is studying the effects of dietary supplements on frog fitness. She hopes to give these captivating critters a fighting chance at survival, and save the corroboree frog for future generations.

Corroboree frogs, found in patchy sub-alpine regions in New South Wales and Victoria, have evolved several unique adaptations to their cold and harsh environment. They hibernate in winter, when temperatures reach below zero, and have a very slow life cycle: They don’t reach sexual maturity until four years of age.

Unlike most frogs, corroboree frogs don’t have webbed toes, and prefer to walk rather than jump. Whereas other poisonous amphibians, such as poison dart frogs, obtain their toxins from the food they ingest, corroboree frogs can also produce their own toxins. It is due to the deadly nature of this toxin that they are so brightly coloured: The frog’s yellow and black stripes warn would-be predators that to eat them would be a fatal mistake.

 
  PhD student Emma McInerney with a southern corroboree frog (left), and a captive-bred corroboree frog at Healesville Sanctuary (right).  © Emma McInerney;  mcdexx/Flickr  (CC BY-NC 2.0)

PhD student Emma McInerney with a southern corroboree frog (left), and a captive-bred corroboree frog at Healesville Sanctuary (right). © Emma McInerney; mcdexx/Flickr (CC BY-NC 2.0)

 

The decline in corroboree frog populations has been drastic and brutal, thanks to a disease called chytridiomycosis. The chytrid fungus, which was accidentally introduced to Australia in the 1970s, attacks the frogs' skin. Although all amphibians have lungs, they also rely on their moist skin to absorb oxygen, and the fungal spores cause the epidermis to roughen and slough off. The outcome for many frogs is almost always a slow, painful death, as they literally wither away trapped in their own suffocating body.

The population crash has been horrendous for both corroboree frog species, but has been worst in the southern frogs. In the last 40 years, the wild population of southern corroboree frogs has declined by an estimated 99%. Currently, there are fewer than 100 individuals left in the wild.

Unfortunately, they are not alone: According to the International Union for Conservation of Nature, amphibians are among the most threatened taxonomic group of animals in the world. It is estimated that 41% of amphibian species are either threatened or endangered, and amphibians in general are in serious decline.

However, many Australians have come to the aid of this winsome native animal, and captive breeding programs are underway at Taronga Zoo, Zoos Victoria and the Amphibian Research Centre. The breeding programs are designed to maintain the species’ genetic diversity, and reintroduce healthy individuals into the wild to bolster the struggling wild population.

So far the programs have been a huge success, and the future for the frogs is looking much brighter. Still, captive breeding is a tricky business. Breeding animals in captivity, where there are no selective pressures such as predators and disease, means the animals might not be adapted for survival when they are released into the real world. To ensure their survival, researchers are constantly looking to improve captive breeding process, to give the frogs a fighting chance.

This brings us back to Emma McInerney’s work, on how dietary supplements might affect the frogs’ survival chances. Specifically, McInerney is focussing on carotenoids, pigments produced by plants that are important for animal health. Carotenoids give carrots their orange, and flamingos their pink hues, and have been linked to important biological functions like growth, development and skin bacterial assemblage. McInerney is hoping to determine whether the addition of carotenoids to a captive frogs’ diet gives them an advantage when they are introduced to the wild.

So far, McInerney has examined the effects of carotenoid supplies on predator avoidance responses. For this experiment, she exposed captive frogs to a simulated predator attack, a fake snake moving on a motorised pulley system. “When thinking about experiments, especially behavioural experiments, I try to think outside the box,” says McInerney. “Being a biologist requires a lot of creativity.”

 

A corroboree frog escapes from a simulated predator. Video by Emma McInerney

 

What she has found is that carotenoids do indeed have a positive effect: Frogs that received additional carotenoids in their diet responded better to the simulated predator attack by moving further away from the ‘predator’. In the wild, this kind of improved reaction could be the difference between life and death.

Other researchers in McInerney’s lab have found that carotenoids also have a positive effect on the frogs' colouration and skin bacterial assemblage.

“These findings are really exciting because we now know that carotenoids have positive effects on the fitness of these frogs, which could have future implications for captive breeding and reintroduction programs,” says McInerney.

From here, McInerney will observe the survival rates of reintroduced frogs in a wild setting, to see if the benefits of carotenoids persist after the frogs have been released into their natural habitat.

For a species with such a small population, every individual counts. If dietary supplements prevent these precious frogs from becoming snake meals, then that is vital information for conducting captive breeding programs. For southern corroboree frogs, carotenoids might just be the leg up they need to survive and recover in the wild.

Edited by Andrew Katsis