Hundreds of endangered Australian Green and Golden Bell found a cozy and toasty place to huddle up. Unbeknownst to them, this black brick has been set up especially for them. This “sauna” is a refuge, a treatment center where heat does the healing.
These makeshift “frog saunas,” made from common ten-hole masonry bricks and warmed by the sun, are giving conservation biologists a rare dose of good news in the long war against one of nature’s most devastating pandemics: chytridiomycosis, caused by the fungus Batrachochytrium dendrobatidis (or Bd). For decades, this skin-eating pathogen has swept across the globe, killing off amphibians at an unprecedented scale and altering entire ecosystems.
A frog crisis
Once abundant across Australia’s southeast, the green and golden bell frog (Litoria aurea) is now teetering on the edge. This striking amphibian once thrived in urban ponds and coastal wetlands. But over the past few decades, its populations have plummeted — more than 90% of its range has vanished — largely due to habitat loss and the relentless spread of Bd.
First identified in the 1990s, this condition has been implicated in the decline of over 500 amphibian species and at least 90 confirmed extinctions. The fungus targets a frog’s skin — essential for respiration and hydration — causing lethargy, heart failure, and death.
“This is the worst pathogen ever”, says Macquarie University biologist Anthony Waddle.”Nothing has ever caused this much devastation,” he told AFP. “In Australia, we have frogs that only live in glass boxes now. This is a huge, ongoing problem.”
Some amphibians, such as the American bullfrog, can carry Bd without showing symptoms, turning them into mobile disease vectors. Others, like the green and golden bell frog, are set for extinction if we don’t act. Once widespread across southeast Australia, this striking frog has disappeared from over 90% of its range.
It’s so bad that researchers are forced to take last-ditch, desperate attempts to protect species.
“The last line of defence is bringing the frogs into captivity where you can cure and protect them. We’re slowly watching species blink out.”
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Heat can help
In the wild, some frogs bask in warm microhabitats that keep Bd at bay. But natural hotspots are rare and inconsistent. That’s why Waddle’s team built sun-warmed shelters—essentially stacks of black bricks placed in small greenhouses. When exposed to the elements, these structures reach internal temperatures that are lethal to the fungus but tolerable for the frogs.
First, the team wanted to see whether frogs would even go in. Frogs were given a choice between shaded and sun-exposed greenhouses. The results were unambiguous: frogs overwhelmingly chose the hotter shelters and were 3.1 degrees Celsius warmer, on average, than their peers in the cooler bricks.
Then, the team wanted to assess effectiveness against infection: the setup proved strikingly effective. Frogs exposed to Bd but given access to these hot refuges were not only able to clear their infections, they also developed resistance. In follow-up experiments, these “pathogen-experienced” frogs were 23 times more likely to survive a second encounter with the fungus than naïve frogs that had received only heat treatments.
At the same time, the shelters didn’t stress the frogs. There were no observed differences in weight or breeding timing between the groups.
All in all, the approach proved remarkably effective.
This isn’t a panacea
The frog sauna isn’t a universal cure, and it won’t work for every amphibian. While the green and golden bell frog thrived in the heated bricks, other species may not fare as well. Some frogs, especially those adapted to cooler, shaded environments, may avoid the warmer conditions altogether. For them, the heat might be too much to handle.
Indeed, past attempts to use heat-based treatments for other species, including the critically endangered Panamanian golden frog, have failed. The fungus is simply too widespread, too resilient. It thrives in cool, moist environments, spreads through water, and clings to amphibian skin. Even chemical treatments and antifungal drugs — while effective in lab settings — are nearly impossible to deploy in the wild at scale. Vaccines made with weakened strains of the fungus have shown some promise, but these, too, are largely limited to captive breeding programs.
That’s why the frog sauna is so important: it can work as a stopgap. By helping frogs clear infections and survive long enough to breed, the saunas may offer more than just temporary relief. They could buy time for natural selection to do its work.
Some frog species have already shown signs of evolving resistance. The idea is that with each generation, the survivors might pass on traits that make them more resilient to the pathogen. But that only works if the frogs live long enough to reproduce — and in the depths of a chytrid outbreak, that’s a tall order.
The idea may be simple, but the implications are profound. If thermal shelters can be adapted for other warm-loving species, they could offer an accessible and low-cost conservation tool in places where more intensive interventions aren’t feasible. Parks, reserves, and even private landowners could install the bricks.
It’s cheap, it’s scalable, and the frogs like using them. The frog sauna doesn’t promise to save them all. But it does give one battered species a second chance — one warm brick at a time.
The study was published in Nature.