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.