A deadly fungal disease has ravaged the world’s amphibians for decades, 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 species of amphibians and, according to one estimate, contributed to the decline of hundreds more.
“Chytrid is this unprecedented wildlife pandemic,” said Anthony Waddle, a conservation biologist at Macquarie University in Sydney, Australia. “We’re watching species and populations flicker.”
But like many formidable foes, the chytrid has an Achilles’ heel. The fungus that is the primary culprit – known as Batrachochytrium dendrobatidis or Bd – thrives in cold weather and cannot withstand the heat.
Now, a new study provides evidence that conservationists might be able to keep the fungus at bay by providing the frogs with a warm place to survive the winter. Researchers have found that a simple pile of sun-warmed bricks attracts the green-and-gold bluebell, a vulnerable Australian species. These thermal shelters increase the frogs’ body temperature, helping them fight off fungal infections and possibly preparing them for long-term survival.
“If we give frogs the ability to rid themselves of infection with heat, they will,” said Dr. Waddle, first author of the new paper, which was published Wednesday in Nature. “And they will likely be resilient in the future.
The green and gold bell, formerly found 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, the chytrid often flares up in winter and early spring, when daytime temperatures can reach highs 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 to areas that were an average of 84 degrees Fahrenheit warmer than is ideal for Bd.
In a second experiment, the researchers placed frogs infected with fungi in different climates. Some frogs spent weeks in the relative cold, in habitats set to 66 degrees. These frogs hid large amounts of mold for weeks. Over the following months, more than half of them died, said Dr. Waddle.
The researchers found that frogs placed in warmer environments or with access to a wide range of temperatures recovered quickly from their infections.
Frogs that were cured of chytrid using this kind of “heat treatment” were also less susceptible to the disease in the future. When re-exposed to Bd six weeks later – without the benefit of a hot environment – ​​86 percent of them survived, compared to 22 percent of frogs that had not been previously infected.
Finally, the researchers tested these findings in large outdoor spaces that more closely resembled real-world conditions. The researchers stacked several holes of drilled bricks in each enclosure and covered each pile with a small greenhouse. The greenhouses were exposed to the sun in half of the enclosures and shaded in the rest.
They then 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 infection.
Shaded and unshaded shelters attracted frogs, which took up residence in the holes inside the bricks. The researchers found that frogs with access to sun-heated bricks maintained body temperatures roughly six degrees higher than frogs in the shade. This increased temperature was enough to reduce the amount of mold that the frogs harbored. “Just a difference of a few degrees can tip the scales for the frogs,” said Dr. Waddle.
Frogs that survived previous encounters with the chytrid also had relatively mild infections, the researchers found, even though they were not allowed access to sun-warmed shelters.
The results suggest that thermal shelters could act as a kind of “crude immunization,” said Dr. Waddle, helping frogs survive their first bout with Bd and leaving them less susceptible in the future. “Then you seed the population with resistant frogs that would reduce the population level of the chytrids.”
This strategy won’t work for all endangered amphibians—not all of them seek warmth, for one—but it could be a low-cost intervention that benefits many, said Dr. Waddle, who hopes to test this approach with other frog species.
Meanwhile, he installed shelters at Sydney Olympic Park, which is home to a wild population of frogs. He’s also getting the public involved, encouraging local residents to “build a frog sauna,” he said. “We’re trying to get people to put them in their backyards.”“