Australia isn’t the first place that comes to mind when people think of amphibians. Most of this vast continent is dry, but its eastern regions harbor temperate and subtropical rainforests as well as woodlands of conifers and tree ferns. It’s prime habitat for frogs. Amphibians here followed their own evolutionary paths during Australia’s millions of years of isolation, making them not only biologically unique, but also of intense interest to science. It’s also here that the great amphibian decline has taken some of its greatest tolls.
Most frogs spawn their eggs in masses of protective jelly, in which they mature into tadpoles. But Queensland’s gastric-brooding frogs (Rheobatrachus
spp.) that now may be extinct, swallowed their eggs once they were fertilized, and they developed in the mother’s stomach. Hormones from the eggs stimulated the frog’s body to stop producing stomach acid, and her entire digestive system shut down while her babies grew. After seven weeks, the eggs transformed into tiny froglets, and the mother regurgitated up to 25 young. No other amphibian known has devised such a novel way to reproduce. Scientists have been trying to understand how these frogs turn off their digestive system so effectively, thinking of possible medical therapies for patients with stomach ulcers. Sadly, they may never know.
Only two species
of gastric-brooding frogs were ever known: northern (R. vitellinus
) and southern (R. silus
) varieties that made up the entire family Rheobatrachidae. The southern species was discovered 2,600 feet up in Queensland’s Blackdall and Conondale mountains in 1972, and four years later a population of about 78 was reported there. But in 1979, the species suddenly began disappearing, and by 1981 only one was found. Since then R. silus
has vanished completely and it is presumed to be extinct. In 1984, a northern species was discovered high in the undisturbed mountain rainforests of Queensland’s Eungella National Park and at the time was common in the region’s streams. Populations seemed strong in March 1985, but two months later every frog had disappeared. Despite extensive searching, none have been found since.
In the misty, moss-covered bogs of the Snowy Mountains in New South Wales lives an equally unique species called the Corroboree frog (Pseudophryne corroboree
). This small black-and-yellow striped amphibian has become a star among scientists for a bit of evolutionary wizardry it alone can perform.
Until recently, toxicologists were confident that the only creatures able to create their own defensive poisons were invertebrates and plants. Vertebrates, like the famous “poison dart” frogs of South America, acquired their toxins from creatures they ate, sequestering the chemicals in their bodies for use against predators. Scientists thought this was true for the Corroboree frog too, but in 1980 pharmacologist John Daly from the National Institutes of Health in United States discovered that it secreted two different nerve-poisons from its skin.
One was a toxin common in many Australian frogs, but the second was a compound unique to P. corroboree
. In the lab, Daly found that the common poison was indeed derived from the frog’s dinner, but the other was produced by the Corroborees themselves—a vertebrate first! Since then, scientists have been trying to figure out how these amphibians make their poison and how it might be used against chronic pain or heart arrhythmias in people.
Once again, time may be running out for these amazing animals. The CI-funded Global Amphibian Assessment reported that populations of three Pseudophryne
frogs, including P. corroboree
, have declined precipitously in recent years. Today, the Corroboree frog is Critically Endangered with fewer than 250 animals remaining in the wild.
So far, no one has found a conclusive reason why these frogs are dying. Theories abound, but most scientists agree that the three most likely suspects are pollution, habitat destruction, and the chytrid fungus that has decimated amphibian species around the world. What seems to implicate disease as an important cause is the speed at which some frog populations in eastern Australia have crashed.
While captive-breeding programs for Corroboree frogs are ongoing, returning them to the wild and exposing them to chytridiomycosis could be a death sentence. No one knows how many other unique amphibian species have been lost. Only further research into amphibian disappearances, and the biology of the chytrid fungus, will provide answers on how to protect those dwindling amphibians still clinging to existence.
CI’s Amphibian Action Fund (AAF) which supports this kind of basic research around the world in biodiversity hotspots, is currently working with leading herpetologists to identify threatened species like the Corroboree frog, and to help protect their habitat.