Frog saunas could help the amphibians cope with a deadly fungus

Green and gold bell frogs in artificial hotspot shelter. Credit: Anthony Waddle

Green and golden bell frogs used to be so common in Sydney, Australia that residents would find them hiding in their mailboxes and toilets. “They’d be everywhere,” says Anthony Waddle, a biologist at Macquarie University in Sydney. In the 1960’s though, the population started to slump. More recently, it’s plummeted. Surveys from the past 15 years indicate the species has disappeared from 90% of its range. They’re now listed as endangered in New South Wales, with numbers that continue to decline. Globally, the green and golden bell frog is considered ‘Near Threatened’ on the International Union for the Conservation of Nature’s Redlist. 

There are many contributing factors to the bell frogs’ decline: Invasive fish introductions, habitat loss, cats, and water pollution are some examples. One of the biggest ongoing and difficult to manage threats is chytridiomycosis (chytrid, for short), a fungal disease caused by Batrachochytrium dendrobatidis (Bd). Chytrid was first detected by scientists in Queensland, Australia in 1993–though the disease had been present in the country since at least 1978. Since its initial discovery, the disease has spread to every continent except Antarctica, infecting over 500 amphibian species and driving at least 90 species into total extinction. It is the deadliest known wildlife pathogen.

Amid the devastation, there’ve been few hopeful findings. But a deceptively simple solution could help solve the problem of startling declines in some species. Low-tech frog ‘saunas’ enable infected green and golden bell frogs to recover from chytrid and develop immunity to the subsequent Bd exposures, according to a study led by Waddle and published June 26 in the journal Nature. 

The research “was all about coming up with a practical way we can manage this really nasty disease in nature,” Waddle tells Popular Science. Bd is a cold-loving fungus that stops growing at around 82 degrees Fahrenheit and dies at 86 F, he explains. Amphibians able to withstand heat can clear chytrid infections when kept warm for long enough. In Australia and other places with pronounced seasons, winter is generally the worst time for chytrid-related losses.

For over a decade, biologists have observed that some frog populations with access to warm places of refuge within their habitat withstand chytrid better. One 2011 study noted that boulders, heated by the sun in a waterfall clearing, were likely allowing critically endangered species to persist. The new study finds encouraging evidence that artificial “hotspot shelters,” built from brick and plastic tarp, could do the same thing. 

“It’s a very simplistic but also elegant experiment,” says Ana Longo, a disease ecologist and evolutionary biologist studying amphibians at the University of Florida who was not involved in the new research. “The beauty of this experiment is that it could be [widely deployed],” she says. “Many species might possibly benefit from this. It’s very cheap to do, and easy. This could be easily implemented by wildlife managers and conservation agencies,” Longo adds.  

The first step to building an effective amphibian sauna was determining the green and golden bell frogs’ preferred temperature. Using a piece of metal gutter with one hot and one cold side, Waddle and his colleagues established that the species seems to gravitate towards a balmy 84 F, on average–perfect for warding off chytrid, if given the chance. 

Then, the researchers needed to confirm that basking at the preferred temperature would actually help infected frogs. They found that, with access to a thermal gradient habitat and allowed to choose where to be, frogs infected with chytrid effectively recovered within days. They measured infection intensity by swabbing the frogs’ skin and testing for fungal DNA. Six days after being moved into the thermal gradient habitat, the 20 frogs tested had an average infection intensity of zero. 

The scientists further tested if recovering from one chytrid infection helped the frogs survive subsequent exposures. In an experiment of 55 frogs, they found that a previously recovered subgroup was about 23 times more likely to survive a second exposure than a naive control group. 

Finally, Waddle and co took their experiment outside, into 12 different semi-natural mesocosms enclosed by netting and containing ponds with gravel, artificial vegetation, and various shelters. In each mesocosm, they placed a pile of bricks enclosed in a small plastic greenhouse. Each mesocosm housed 19 or 20 frogs exposed to chytrid. They compared mesocosms with shaded greenhouses to those with greenhouses exposed to full sun, and found that in mesocosms with the unshaded, hotter shelters frogs had significantly lower infection intensity over the duration of the 15 week experiment. 

The temperature difference between the two greenhouse treatments was less than 10 degrees F. The shaded greenhouses were still warm. But the difference in infections between the two groups was, at various points, 100 times worse for frogs with access to the marginally cooler structures, says Waddle. In both set-ups, frogs with a prior chytrid infection and recovery fared better against new infection. “As scientists, we always think ‘it’s not going to work,’” Waddle says–so he was surprised and excited to see just how clear the results were.

Other studies have proposed interventions like clearing trees to sunlight ponds and even actively heating water to mitigate chytrid’s effects. However, “this is the first study to put forward a easy to implement and minimally invasive method for providing artificial thermal refugia to amphibians to help them fight chytrid infections,” says Erin Sauer, one of the study authors and a wildlife disease ecologist at the University of Arkansas. Compared with removing vegetation or putting heaters in ponds, the basic brick and tarp greenhouses are cheaper and unlikely to have serious negative impacts on other organisms sharing the ecosystem with frogs. 

Now, conservationists trying to save the green and golden bell frog have an emerging evidence-based tool at their disposal. Waddle hopes to encourage residents of Sydney to build the structures (which cost about $50 to make) in their backyards in a citizen science effort. And in a follow-up experiment, he and his colleagues have already placed 50 of the greenhouses around the Brickpit at Sydney Olympic Park–one of the few remaining sites with a population of the amphibian. Over the next two years, they will monitor the site to see if the dwindling frog numbers there bounce back, if any other species take advantage of the structures, and/or if the shelters cause problems. For instance, they don’t want to end up “creating a snake buffet,” says Waddle. Pending those results, he hopes to test the concept in other amphibians with similar thermal biology. 

Yet there are limits to what frog saunas can do. In significantly cooler places, the greenhouses might not become warm enough to make a difference. And so far, the concept has only been tested in one species. Amphibians vary widely in their temperature preferences and tolerances. Alpine and otherwise cool-adapted frogs like the Sierra Nevada yellow-legged frog in California, which has been decimated by chytrid, would be unlikely to benefit from hotboxes. Fully aquatic salamanders would also be impossible to treat this way, notes Longo. And some of Sauer’s past research even indicates that, for certain species, warmer temperatures can worsen chytrid infections. “Extrapolating this into every other species that’s impacted by cancer, would be really irresponsible,” says Waddle. 

But for green and golden bell frogs, and likely others, bricks and a plastic cover could be a boon in a sea of threats. “It takes this type of bold idea,” to make a difference, says Longo. Many scientists might see an infected and declining population, and just give up, she says. This study offers an alternative–an active response, through which we might “be able to guarantee the persistence of a species.” 

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