Bats With White-Nose Syndrome Choose Habitats Where Disease Thrives

About 15 years ago, the first case of white-nose syndrome was discovered in bats. It appeared in caves near Albany, New York, where explorers spotted the animals with what looked like white powder on their noses. The fungal disease grows in damp, dark places, affecting bats when they are hibernating.

Bats roosting in the warmest sites are most affected because the fungus that causes the disease is able to grow more easily on their skin. Yet many bats keep choosing the less than desirable environments each year, a new study finds.

Instead of moving to a new habitat where their chances of survival are higher, the bats mistakenly choose suboptimal locations where the fungus thrives and the bats often die. Researchers point to this as an example of an infectious disease creating an “ecological trap” for wildlife, where habitat preference and fitness are mismatched.

Researchers working on this study had been following populations of little brown bats (Myotis lucifugus) in Michigan and Wisconsin since 2012, before white-nose syndrome reached those states. This let them see if their hibernation location preferences changed once the fungus took hold. 

“The warmer sites allow the fungus to grow faster on the bats; the faster the fungus grows, the more fungus they have on them, and that causes more pathology and disease,” lead author Skylar Hopkins, a previous postdoctoral scholar at Virginia Tech and now assistant professor at North Carolina State University, explains to Treehugger.

For the study, researchers captured bats and banded them, and then tried to capture them again later. They used swabs to measure the fungal loads on each bat and a laser thermometer to measure the temperature on the rocks next to each bat.

They visited the area twice a year: early in hibernation after all the bats had settled down for the winter, and then again in late hibernation, before the bats had emerged from their hibernation habitat.

Researchers found that bats roosting in warmer sites had larger increases in fungal loads on their bodies from the beginning to end of hibernation (from fall to spring). They discovered that the bats that roosted in warmer areas were more likely to disappear before the late hibernation surveys so researchers were unable to measure and track them.

“We think those missing bats were emerging early due to disease-induced starvation and probably died on the landscape, because there aren't any bugs available for bats to eat in Michigan and Wisconsin before March,” Hopkins says.

They found that more than 50% of bats were choosing to roost in warmer sites, even though they had access to colder, safer locations.

The study findings were posted in the journal Nature Communications.

A Focus for Conservationists

Researchers aren’t sure why bats don’t learn to avoid the more dangerous, warmer sites and instead roost in the safer, cooler locations.

“We expect that bats are physiologically constrained to a narrow range of temperatures that help them to survive hibernation,” Hopkins says. “The warmer sites might have been great for them before the fungus that causes the disease invaded the United States, so the bats recognize those as good sites. But now that the fungus is here, they're deadly.”

Using the knowledge that bats prefer sites that cause higher mortality rates, researchers suggest that the findings may be helpful for conservationists. But it’s not as simple as closing off the warmer locations so bats will instead gravitate toward the cooler ones. There isn't a one-size-fits-all recommendation, Hopkins says. 

“Since we know that bat survival is lowest in the warmest sites, it's true that we should focus carefully on those sites and carefully consider how best to help the bats there. Maybe those sites should be high priority for treating the environment, modifying temps in the sites (especially man-made sites like mines), or yes, maybe even blocking off the sites,” she says.

“But we need to remember that other bat species and other wildlife also use those sites, so we need to balance impacts on those other species with benefits to little brown bat populations. In general, we should just be doing everything we can to be conserving winter and summer bat habitats so that the surviving individuals have the best chances to keep surviving.”