Localized heating of caves during hibernation may reduce bat mortality
White-nose syndrome (WNS) is a poorly understood condition that, in the two years since its discovery, has spread to at least seven northeastern states and killed as many as half a million bats. Now researchers have suggested the first step toward a measure that may help save the affected bats: providing localized heat sources to the hibernating animals.
“We have no idea why it’s spreading so rapidly,” says Justin Boyles, a graduate student in biology at Indiana State University and the first author of the paper, published this week in Frontiers in Ecology and the Environment e-View.
The syndrome has baffled scientists since its discovery in the winter of 2006 in upstate New York, where hibernating bats were found with a mysterious white fungus growing on their faces and wing membranes. Hundreds of emaciated bats were found dead in and around their caves, suggesting that they had starved to death during their hibernating months, and affected populations commonly suffer 75 to 100 percent mortality.
The origins of WNS are virtually unknown – scientists just identified the fungus species last month. But they are still mystified by its relationship to such unprecedented bat mortality.
Boyles and his coauthor Craig Willis of the University of Winnipeg tested the idea, suspected by many in the bat research community, that the fungus causes bats to spend more time out of hibernation during the winter. Mammals must rouse from hibernation periodically, but doing so too often or for long periods of time is energetically costly. When they rouse, the bats must use body energy to keep warm; spending too much time out of hibernation may deplete their fat reserves and cause them to starve to death, say the authors.
Because of the rapid spread of the fungus and the fact that field experiments can take months to years to complete, Boyles and Willis instead created a mathematical simulation to test the idea that the fungus is causing bats to spend more time out of hibernation. Their model took into account the patterns of arousal, body mass and percentage body fat of a particular species, called little brown bats, which are affected by the fungus.
The simulation showed that the patterns and proportion – about 82 percent – of bat mortality observed in affected populations in the wild are consistent with a large increase in the amount of time spent out of hibernation during the winter months. Their results, they say, provide evidence that the fungus is likely affecting bat hibernation patterns.
The researchers then took the simulation one step further. They reasoned that one way to help affected bats save their energy reserves and survive the winter is to provide them with a heat source, so they don’t have to create as much body heat when they rouse. Bats often fly to the warmest parts of their cave during bouts of arousal.
“They already do this in the wild,” Boyles says. “What we’re suggesting is accentuating that behavior.”
When the authors altered the simulation to include localized heat sources the bats could gather in during arousals, the model showed that mortality levels dropped to as little as 8 percent.
These results could be used in the short-term to prevent bat populations from crashing below sustainable levels, the authors say. They are currently developing a system, using wooden boxes and heating coils, to create warm pockets in bat caves. The plan holds no guarantees: the overall temperature in the cave needs to stay cold enough so that bats can still lower their body temperatures during hibernation. But the researchers are optimistic.
“By insulating the bat boxes and carefully selecting where we will place them, we think we can solve this issue,” says Willis.
Further, saving afflicted animals may not be sustainable in the long term, say the authors. If WNS is transmitted in spring and summer by surviving bats, saving its carriers will also save the disease, they write. At present, the search for a remedy for this mysterious fungus continues to stymie scientists.
“I can’t even guess what the cure or the solution to this is going to be,” says Boyles. “This isn’t a cure. We’re going for a stopgap.”
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