For decades, a deadly fungal disease has stalked 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 caused the extinction of at least 90 amphibian species and contributed to the decline of hundreds of others, according to one estimate.
“Chytrid is this unprecedented wildlife pandemic,” said Anthony Waddle, a conservation biologist at Macquarie University in Sydney, Australia. “We’re looking at eye-opening species and populations.”
But, like many formidable enemies, the chytrid has an Achilles’ heel. The fungus that is the main culprit – known as Batrachochytrium dendrobatidis, or Bd – thrives in cool weather and cannot withstand the heat.
Now, a new study provides evidence that conservationists may be able to keep the fungus at bay by giving the frogs a warm place to hibernate. A simple pile of bricks warmed by the sun, researchers found, attracts the green and golden bell frog, a vulnerable Australian species. These thermal shelters raise the frogs’ body temperatures, helping them beat fungal infections and, possibly, setting them up for long-term survival.
“If we give frogs the ability to clear their infections with heat, they will,” said Dr. Waddle, first author of the new paper, which was published Wednesday in Nature. “And they are likely to be resistant in the future.”
The green and golden bell frog, once common in south-eastern Australia, has disappeared from much of the landscape and is now listed as endangered in the state of New South Wales.
In Sydney, home to some of the remaining bell frogs, chytrid often flares up in winter and early spring, when daytime temperatures can top 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 milder climates 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 frogs infected by the fungus in a variety of climates. Some frogs spent weeks in the relative cold, in habitats set at 66 degrees. Those frogs maintained high levels of the fungus for weeks. In the months that followed, more than half of them died, said Dr. Waddle.
But frogs placed in warmer environments, or given access to a wide range of temperatures, quickly recovered from their infections, the researchers found.
Frogs that were cured of chytrid, with the help of this type of “thermal treatment”, were also less susceptible to the disease in the future. When they were re-exposed to Bd six weeks later—without the benefit of a heated habitat—86 percent of them survived, compared with 22 percent of frogs that had not been previously infected.
Finally, the researchers put these findings to the test in large outdoor environments that more closely resembled real-world conditions. The scientists piled several bricks filled with holes 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 other half.
Then, they released a variety of frogs into each enclosure. Some of the frogs had never been exposed to Bd before, while others had been actively infected with the fungus or had previously survived an infection.
Shaded and unshaded shelters each attracted frogs, which returned home to holes inside the bricks. But frogs that had access to sun-warmed bricks maintained body temperatures that were roughly six degrees higher than frogs that had shaded shelter, the scientists found. This increase in temperature was sufficient to reduce the amount of fungi harboring the frogs. “Just a few degrees of difference can affect the scales for frogs,” said Dr. Waddle.
Frogs that had survived previous chytrid encounters also had relatively mild infections, the researchers found, even when they weren’t given access to sun-warmed shelters.
The results suggest that thermal shelters can act as a kind of “crude immunization,” said Dr. Waddle, helping the frogs survive their first encounter with Bd and leaving them less susceptible in the future. “Then you’re seeding the population with resistant frogs that would reduce the chytrid population level.”
The strategy won’t work for every threatened amphibian — not all are heat seekers, for one — but it could be a low-cost intervention that pays off big, Dr. Waddle, who hopes to test the approach with others. frog species.
Meanwhile, he has installed shelters in Sydney’s Olympic Park, which is home to a wild population of frogs. He’s drawing in the public, too, encouraging local residents to “build a frog sauna,” he said. “We’re trying to get people to put them in their yards.“