If You Give a Frog a Sauna, It Might Fight Off a Deadly Fungus

For decades, a deadly fungal disease has been stalking the world’s amphibians, wiping out frogs, toads and salamanders from the mountain lakes of the United States to the rainforests of Australia. The disease, known as chytridiomycosis, or chytrid, has driven at least 90 species of amphibians extinct and has contributed to the decline of hundreds more, according to one estimate.

“Chytrid is this unprecedented pandemic of wildlife,” said Anthony Waddle, a conservation biologist at Macquarie University in Sydney, Australia. “We’re watching species and populations blink out.”

But, like many formidable foes, chytrid has an Achilles’ heel. The fungus that is the primary culprit — known as Batrachochytrium dendrobatidis, or Bd — flourishes in cool weather and cannot withstand heat.

Now, a new study provides evidence that conservationists might be able to keep the fungus at bay by giving frogs a warm place to ride out the winter. A simple pile of sun-warmed bricks, the researchers found, attracts the green and golden bell frog, a vulnerable Australian species. These thermal shelters boost the frogs’ body temperatures, helping them beat back fungal infections and, perhaps, setting them up for long-term survival.

“If we give frogs the ability to clear their infections with heat, they will,” said Dr. Waddle, the first author of the new paper, which was published Wednesday in Nature. “And they’ll likely be resistant in the future.”

The green and golden bell frog, which used to be common in southeastern Australia, has disappeared from much of the landscape and is now listed as endangered in the state of New South Wales.

In Sydney, where some of the remaining bell frogs reside, chytrid often flares up in the winter and early spring, when daytime temperatures may max out in the 60s. In the first of several experiments documented in the new paper, Dr. Waddle and his colleagues found that the frogs preferred balmier climes when they were available. When placed in habitats with a temperature gradient, the frogs gravitated toward areas that were 84 degrees Fahrenheit, on average, warmer than is ideal for Bd.

In a second experiment, the researchers placed fungus-infected frogs in a variety of climates. Some frogs spent weeks in the relative cold, in habitats set to 66 degrees. Those frogs harbored high levels of fungus for weeks. Over the months that followed, more than half of them died, Dr. Waddle said.

But frogs housed in warmer environments, or given access to a wide array of temperatures, rapidly recovered from their infections, the researchers found.

Frogs that recovered from chytrid, with the help of this kind of “heat treatment,” were also less susceptible to the disease in the future. When they were exposed to Bd again six weeks later — without the benefit of a hot habitat — 86 percent of them survived, compared with 22 percent of the frogs that had not been previously infected.

Finally, the researchers put these findings to the test in large outdoor enclosures that more closely resembled real-world conditions. The scientists stacked some hole-riddled bricks in each enclosure, covering each pile with a small greenhouse. The greenhouses were exposed to the sun in half of the enclosures and shaded in the rest.

Then, they released an assortment of frogs into each enclosure. Some of the frogs had never been exposed to Bd before, while others were actively infected with the fungus or had previously survived an infection.

The shaded and the unshaded shelters each attracted frogs, which made themselves at home in the holes inside the bricks. But the frogs with access to the sun-warmed bricks maintained body temperatures that were roughly six degrees higher than frogs given shaded shelters, the scientists found. That elevation in temperature was enough to reduce the amount of fungus the frogs were harboring. “Just a few degrees difference can tip the scales for the frogs,” Dr. Waddle said.

Frogs that had survived previous encounters with chytrid also had relatively mild infections, the researchers found, even when they were not given access to the sun-warmed shelters.

The results suggest that thermal refuges might act as a sort of “crude immunization,” Dr. Waddle said, helping frogs survive their first bout with Bd and leaving them less susceptible in the future. “Then you’re seeding the population with resistant frogs that would drive down the population level of chytrid.”

The strategy won’t work for every threatened amphibian — not all of them are heat-seeking, for one — but it could be a low-cost intervention that benefits many, said Dr. Waddle, who is hoping to test the approach with other frog species.

In the meantime, he has installed the shelters at Sydney Olympic Park, which is home to a wild population of the frogs. He’s enlisting the public, too, encouraging local residents to “build a frog sauna,” he said. “We’re trying to get people to put them in their backyards.”