Hibernation devastation: White-nose syndrome and our bats

A video camera pans the mouth of Aeolus Cave in Vermont. Limestone rock slabs angle downward into knee-deep snow pack. It zooms in on a handful of bats huddled in a crevice, then descends into the cave. Leaf litter is piled up in drifts on the cave floor. The camera zooms in, and suddenly you realize these are not leaves… they are bats: hundreds and hundreds of dead bats. The footage is all the sadder because this cave houses the largest colony of hibernating bats in the northeast.

Little brown bat showing characteristic fungus growth associated with WNS. (Credit: USGS)

This video clip, filmed by CBS news on a Nature Conservancy property last February, was shown at the N.C. Museum of Natural Sciences yesterday during a public lecture on white nose syndrome, an emerging pathogen affecting northeastern cave-hibernating bats. Lisa Gatens, curator of mammals at the museum spoke to colleagues, students and interested public about the documented occurrences of WNS and the extent of current research.

Gatens displayed a map (below) compiled by Bat Conservation International that shows the range of documented caves infected with WNS.  She pointed out that the southernmost occurrence was in Smith County, Virginia which abuts our state line. “I will not be surprised to find it here,” Gatens said. White-nose syndrome has a 90 to nearly 100 percent fatality rate at most sites. The N.C. Wildlife Resources Commission will conduct winter bat surveys in mountainous western North Carolina next January and February, Gatens said.

The bats that are becoming sick are all insect eaters. The largest northeastern colony is thought to eat two billion insects per night. You do the math. Losing these mammals would cause incalculable damage to agriculture crops and could contribute to an uptick in mosquito-borne diseases such as West Nile virus, which infects humans.

Range and spread of afflicted winter bat colonies. (Credit: Bat Conservation International) Click to enlarge

A group of leading U.S. scientists from 12 states and federal wildlife agencies warned in a consensus statement issued in May 2009 that the disease “has caused the most precipitous decline of North American wildlife in recorded history.” Their statement also estimated bat deaths to have exceeded 1 million over a three year period. “If the spread of WNS is not slowed or halted, further losses could lead to the extinction of entire species and could more than quadruple those that are federally listed as endangered in the U.S.,” they wrote. In the U.S., six bat species are susceptible to WNS: the little brown bat, big brown bat, tri-colored bat, Northern long-eared bat, Indiana bat and gray bat. The last two are federally endangered and all six occur in North Carolina.

The exact mechanism that kills the bats is still unknown, but scientists have many leads. The sickness leaves hibernating bats starving in mid-winter, and non-furred parts of their skin and muzzle become coated in white fungi. Their wing membranes become splotchy and scarred; in the later stages, their frayed wings may appear like moth-eaten fabric. The bats burn through their fat reserves too fast and emerge from hibernation too early in a desperate mid-winter search for food, but they often starve and freeze. One line of thought is that animals experience a chemical interference that disrupts their metabolism and hibernation state, causing them to starve. Another line of thought is that the fungal growths irritate their skin, causing them to become active and wake early, Gatens said.

So what is killing our bats? Why are they waking up? Why are they leaving their caves? A cold-loving fungus was consistently found in association with dead or dying bats. While the species is new to scientists — it was formally described in the journal Mycotaxon in the April-June issue, and given the name Geomyces destructans — the genus was all ready known. Species within Geomyces are known to occur in bats in Europe, but European bats have not become sickened by it, Gatens said.

Scientists are scrambling to determine whether the fungus itself is a causal agent or if it is a secondary infection that invades after something else weakens the bat’s immune systems. Gatens said that scientists at the American Museum of Natural History are collecting “wing punches,” tissue samples from bats’ wing membranes, and storing them in liquid nitrogren DNA freezers. In a worse case scenario, these specimens will provide a record of extinction, but at best they will help researchers figure out how to staunch the deaths.

A few factors about the bat’s life history and behavior may affect their outcomes too. The bats return to their same hibernating roost year after year, a habit that could further imperil their resistance to whatever is causing the deadly malady.  It’s also possible that they may be eating something that accumulates in their systems. Gatens said that summer surveys will collect wing punches and hair samples to investigate if pesticides or heavy metals from their insect prey base are building up in their bodies. The fact that the six affected bat species live more than 30 years, on average, means there is plenty of time for biotoxins to accrue, Gatens said.

“One question in my mind is how their lowered fat reserves will affect reproductive rates,” Gatens said, explaining that they typically only have one pup per year. “They mate before hibernation, but one of their survival tricks is that fertilization does not take place until the end of hibernation. And if a female’s fat reserves are lower than normal because of white-nose syndrome then it’s hard to imagine this would not have an effect on gestation.” The bat’s slow reproductive rate will be an obstacle to recovering decimated populations, even without the added burden of disease.

In the question-and-answer portion after the talk, one museum scientist suggested the beginnings of this epidemic paralleled the historic early stages of HIV emergence in people.  Geology curator Chris Tacker suggested that if all the hibernation sites were limestone caves, then acid rainfall could be channeling a chemical change into these geologic fracture zones, speeding the release of carbon dioxide into the cave air by the dissolution of calcium carbonate. “This would mean that the bats would be hyperventilating all winter long, which could explain why they run through their fat stores,” he said. He emphasized this was just a suggestion for the biologists to consider. “The parallel in my mind is the guys who climb Mount Everest. They hyperventilate and loose lots of body fat at elevation where the oxygen is thinner,” Tacker explained. “They don’t sleep well because they are not getting enough oxygen, and they suffer necrotic tissue losses at their extremities.” A similar situation appears to be unfolding with the bats. The possibilities are endless, and the data is still being accrued.

The first signs that something were amiss came from photographs taken by cavers in N.Y. in February 2007. Since then, the number of affected hibernacula – what scientists call groups of bats that huddle close in caves to hibernate through the winter – have rapidly increased and spread south-southwest down the Appalachian Mountain cordillera.

Toward the end of the CBC video clip, Vermont Fish and Wildlife biologist Scott Darling said, “I think of all the work people like me have done to try to conserve these animals and its being taken away so fast… You can’t help but have it tug at your heart, tear at your soul.”

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