A fungus disease has caused declines in over 200 amphibian species. The fungus thickens their skin, keeping them from absorbing nutrients until their hearts stop. Scientists recently developed the first treatment for the chytrid fungus which causes the disease. But there are new types of chytrid that put amphibians in the United States and elsewhere at risk if the fungus arrives on the bodies of amphibians shipped as pets. How well prepared native species are to resist new forms of the disease remains unknown.
But recent ecological detective work into an usually well-hidden salamander suggests at least one species’ behavior is adapting to slow the spread of chytrid. By swabbing live California slender salamanders and museum specimens of the species collected over the last century, a team of scientists now has a better sense of what happened during the disease outbreak and how the disease changed salamanders’ social interactions.
“It’s the same thing you see on CSI when they do a cheek swab and then test for DNA of the criminal—only it’s on the skin of the salamander,” said Carla Sette, a graduate student at the University of California-Santa Cruz and lead author of a paper in the December issue of the journal Biological Conservation.
The team’s analysis revealed that when the criminal in question—chytrid species Batrachochytrium dendrobatidis (Bd)—arrived in California, it invaded rapidly over a few decades. This is important, because in other locations such as Illinois and Brazil, scientists have found that Bd stayed at a steady level for over 100 years, said Vance Vredenburg, biology professor at San Francisco State University and co-author of the paper. But the surveys of living salamanders revealed something more.
California slender salamanders are nocturnal and tend to hide under logs in large clusters, nearly on top of each other, Sette said. “They’re very gregarious. You can flip a log and find almost a dozen under there,” she said.
While hunting for slender salamanders, the researchers uncovered a pattern. In the populations exposed to chytrid for decades, they found smaller group sizes compared to those in areas where chytrid arrived just a few years ago. The researchers think this means the salamanders are spreading themselves out to slow the rate of chytrid transmission.
However, Karen Lips of the University of Maryland, who discovered the chytrid outbreak in Central America, challenged this idea. “I think you have to rule out a lot of other examples to get to that conclusion,” she said. For instance, instead of spreading themselves out farther apart in smaller groups, there could simply be fewer salamanders everywhere, Lips said.
Vredenburg said that unfortunately there are no total population data available for the species. Counting them is not as simple as tallying them up from a helicopter.
“It’s not like counting buffalo,” Vredenburg said. “So that’s a good criticism. But I think it doesn’t take away from the study.” He said that his goal is to expand knowledge of less observable species, since only a minority of amphibian species is active during the day and easily countable.
Furthermore, Vredenburg said he thinks the pattern of group size they observed is “more easily answered with behavior than just with density alone.” Scientists have documented a couple other behavior changes in response to chytrid, he added.
In a lab experiment, toads from Florida that had been infected with Bd before avoided the side of a tank with the fungus present. Golden frogs in Panama exposed to chytrid appear to sun themselves to heat their bodies up, seeming to help protect them from the fungus, which prefers cooler temperatures. If Vredenburg’s hypothesis about the California slender salamander is correct, it could add to a growing picture of how amphibians adapt to fungal disease.
But the question remains—what will happen with vulnerable amphibians if new species of chytrid, such as Batrachochytrium salamandrivorans (Bsal), invades North America?
Vredenburg and his colleagues are currently exposing amphibians to Bsal in the lab to see which species are most susceptible to infection. Still, Vredenburg said the behavioral changes in species like the California slender salamander could help them roll with the next punch from chytrid.
“It’s a really good strategy, and it might actually help them,” Vredenburg said.