Why Are Some Frogs Surviving a Global Epidemic?

A killer fungus could be changing. Or the frogs themselves could be evolving.

The silhouette of a frog sitting on a big green tree leaf
Lev Savitskiy / Getty

In December 2004, Joyce Longcore was dispatched on an unusual mission. At the time, Longcore, a mycologist at the University of Maine, was one of the world’s only experts on a division of fungi called Chytridiomycota, or chytrids. A few years earlier, she had identified a new genus and species of chytrid called Batrachochytrium dendrobatidis—Bd for short—that turned out to be a primary cause of the massive amphibian die-offs that had recently been reported on several continents. In Central America, the Bd outbreak was sweeping east, decimating frog and toad populations along the way. In late 2004, dead frogs began appearing in El Copé National Park, a patch of cloud forest in the mountains of central Panama. Ecologists documenting the declines asked Longcore to go to Panama and collect Bd samples for study.

Working in an island laboratory at the south end of the Panama Canal, Longcore took tiny skin samples from dead frogs found in El Copé, then used them to culture Bd in petri dishes. When she left a week later, she sealed the dishes and stacked about 20 into her carry-on luggage, repeatedly explaining their purpose to bemused airport-security officials. Back in her lab, she froze a set of fungi samples; some were used to trace the genetic origins of the epidemic, while others were stored for future work.

In the years since, amphibian die-offs have continued. An estimated one-third of the world’s nearly 6,000 amphibian species are threatened with extinction, and hundreds more are considered “data deficient,” often because they haven’t been seen for years. Alongside the bad news, though, are scattered reports of reappearances: Populations of frogs and toads thought extinct have been recently rediscovered, in greatly reduced but growing numbers. In a world where accelerating human movements are spreading fungal diseases to new places and new hosts, knowing how and why some amphibians survive Bd could be important to all species—and part of the answer, it turns out, lay frozen in Longcore’s lab.

In 2012, the University of Nevada, Reno, biologist Jamie Voyles, who had studied amphibian declines in Panama for nearly a decade, hypothesized that some frog and toad populations were recovering because the Bd fungus, after years on the loose, had gradually become less deadly. “That’s the conventional wisdom about infectious diseases—that because pathogens generally have shorter life spans, you should see evolution in the pathogen rather than the host,” she says. But as she and her colleagues report on Thursday in Science, they found no significant differences between the decade-old Bd samples from Longcore’s freezer and more recent samples from field sites in Panama: Growth rates, genetics, and other key characteristics all appeared to be the same. “I kept saying, ‘Let’s run it again, let’s do it again,’” says Voyles, “but the more tests we threw at them, the more convinced I became that my original hypothesis was dead wrong.”

If the Bd fungus wasn’t changing, Voyles and her team surmised, perhaps something about the animals themselves had allowed them to survive and start to recover. In lab studies, the researchers found that skin secretions from wild frogs were better able to inhibit Bd than skin secretions from frog populations moved into captivity to protect them from the fungus, suggesting that the wild frogs may have evolved better chemical defenses—or that the captive frogs, protected from pathogens for generations, may have evolved weaker ones.

Fighting off Bd in a petri dish, however, is different than fighting it off in the wild. The University of Maryland ecologist Karen Lips, who has been studying amphibian declines in Central America since the early 1990s, points out that survival varies across the landscape, and can be influenced by any number of factors. It’s possible, for instance, that survival rates increase as numbers drop, simply because animals in a less dense population are less likely to infect one another. “The animals that are hopping around in the forest today are living in an entirely different system—one that has hardly any frogs,” she says.

Voyles agrees that the reasons for survival aren’t simple. “We don’t think that the skin secretions are the only thing that makes a difference for the host,” she says. “It’s probably one piece of a very complex puzzle.”

The possibility that captive-raised frogs may be less suited for survival than their wild counterparts, however, raises questions for amphibian rescue efforts. In the mid-2000s, when Bd first roared through the forests of Central America, a team of ecologists and conservationists decided to create an “ark” for threatened amphibian species, collecting individuals ahead of the epidemic and raising them in captivity in Panama, the United States, and elsewhere. Thanks to such projects, some 55 species of amphibians are now part of rescue programs worldwide. One hope was that the Bd fungus would eventually become less virulent, and that captive animals could be reintroduced. The results suggest that’s not happening anytime soon, at least not in El Copé.

“So now the conservation biologist has a dilemma,” says James Collins, an ecologist at Arizona State University who is deeply involved with amphibian conservation, including captive-breeding efforts. “We know that a handful of these species might be able to recover. If we’re out in the forest and a pair of frogs hops across our path, what do we do? Do we pick them up and put them in a zoo, or do we say ‘You’re on your own’?” In the wild, Collins says, the frogs might well die of Bd. In captivity, however, their descendants might adapt to a Bd-free environment—and have to stay there forever.

Steven Whitfield, a conservation biologist at Zoo Miami who is studying amphibian reappearances in Central America and elsewhere, sees a third option: to breed Bd survivors in captivity, then reintroduce captive animals to wild populations in order to accelerate recovery. While that strategy wouldn’t bring back the hundreds of species that are likely already lost from the wild, it could give surviving species a better chance of withstanding future threats. It’s not exactly good news, says Whitfield, but then again, amphibian experts are used to much worse: “We’ll take all the optimism we can get.”