Web edition: July 5, 2012
Ecologist Kate Langwig of Boston University and her colleagues want Eastern bats to listen up: No more cuddling — at least during hibernation. Just keep those wings to yourselves.
Many populations of hibernating bats have died off shortly after encountering a new, virulent fungal infection known as white nose syndrome. Since the winter and early spring of 2006, huge numbers of hibernating bats have contracted the skin-digesting infection (SN: 9/10/11). To date, Geomyces destructans — the fungus responsible — has claimed the lives of at least 5.7 million to 6.7 million bats, according to the U.S. Fish and Wildlife Service.
Langwig’s team was curious why some affected populations experienced quick and total annihilation while others have hung on, albeit tenuously. Did the different death rates trace to something about the environment of their respective caves? Or did lifestyle render some bats more likely to pick up the disease?
The researchers’ new analysis now finds both factors greatly affect infection risk.
For instance, “we found that [hibernation] sites with the coolest and driest conditions were at least initially thermal refuges from disease,” Langwig says. “And that’s good news,” she adds, because it means that as white nose infection continues to spread, some affected sites will offer winter roosting bats a fighting chance for survival.
Of course, the bats don’t know this, so they’ll just maintain their old behavior, returning to the same hibernation sites that they always have. And Langwig’s team found the barest modicum of good news about this as well. But only a little.
Certain species pack into tight, dense clusters during hibernation. Whether there are 30 bats or 3,000 in a given cave or mine, some species will crowd together cheek by jowl, shoulder to shoulder. These bats face the gravest risk of infection, the researchers report online July 3 in Ecology Letters. Whether their wintering colony is large or small, the infection rate is the same — massive.
The endangered Indiana bat is among species in this category. Its huddling, cuddling nature during hibernation would suggest the species is ripe for epidemic infection. And repeat surveys of this species following initial infection with white nose syndrome showed that death rates “did not ameliorate over time,” even as the population began crashing, Langwig reports.
For bats that prefer to leave a little wiggle room between themselves and hibernating neighbors, white nose risk is lower — and diminishes as a colony’s size shrinks. This suggests that as the infection culls large numbers of these species, such as the tricolored bat, the surviving cohort can eventually stabilize at some much reduced number. If those hangers on eventually develop a resistance to the fungus (as appears to have happened amongst European bats) these colonies may again be able to grow — and again throng in tightly packed hibernating clusters as they once did.
But even among less social hibernators, there’s variation in vulnerability. For instance, the northern long-eared bat is a solitary hibernator. And though the rate of its decline from white nose has slowed as the surviving population has shrunk, there has been no true leveling off. So there appears no minimum size that will permit the infected population to stabilize and hold on, authors of the new study conclude.
Followup surveys of infected communities show 14 northern long-eared populations went locally extinct within two years of white nose syndrome’s arrival, Langwig notes; none remained alive five years after the fungal epidemic first struck their hibernation site.
Of note: One species appears to be bucking the trend for its behavioral “type.” Little brown bats, once the most common species throughout the eastern United States, are known to be gregarious and among those densely packed social hibernators. This explains why most white nose victims have belonged to this species. However, since the fungal infection initially hit back in early 2006, a large share of the survivors have changed their nature. They now hibernate singly.
Where virtually none had spent the winter as loners 10 years ago, Langwig reports that today 75 percent of little brown bats are now roosting individually in some infected caves or mines. And the upshot: “Beginning about four years post-white nose, several populations actually started to grow again.”
Stu Vandermark, a longtime Science News reader can attest to the seeming rebound of little browns. For roughly 15 minutes each night around dusk between June and September, he has been reveling in their aerial acrobatics as these bats flutter overhead — and then dive to harvest insects on the wing.
“It’s like they never suffered any kind of diminishment,” he says. Yet Vandermark knows, first hand, that they have. In recent years, their visits to the tree-lined field behind his home in Framingham, Mass., had declined sharply. By 2009 there were no bats, and in 2010 he saw a single bat on each of just two evenings.
But last year they began to return in force. And now “there are as many bats out there as I’ve ever seen at any time,” he says. “My wife is laughing at me because I’m so ecstatic when I’m out there and see them dancing around. They’re such a joy.”
K.E. Langwig, et al. Sociality, density-dependence and microclimates determine the persistence of populations suffering from a novel fungal disease, white-nose syndrome. Ecology Letters. Online July 2, 2012. doi: 10.1111/j.1461-0248.2012.01829.x
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