Full of dark hollows and snug crevices, caves provide the ideal habitat for bats. But with their low temperatures and humid conditions, there's something else they're perfect for, too: breeding the fungus Geomyces destructans.
Since 2006, some one million bats across six different species have been killed in North America – all as a direct result of white nose syndrome (WNS), a disease brought on by exposure to Geomyces destructans. The fungus, which infects and invades the living skin of hibernating bats, turning their snouts a frosty white, is thought to be transmitted from one cave to the next by people moving between them. In some bat colonies, exposure to the fungus has produced a mortality rate in excess of 95 per cent. It is, says Alan Hicks of the New York State Department of Environmental Conservation, "the gravest threat to bats... ever seen".
And it's not just limited to the United States. In Canada, several fungus-hosting caves have been identified. Affected species include the already-endangered Indiana and grey bats, as well as the little brown bat and the cave bat. In Europe, meanwhile, five different species have been infected – though the mortality rate remains far below that in North America. How long it will stay that way is unclear.
The problem is that, aside from the disease's gravity, scientists know very little about it. It's not yet apparent, for instance, whether Geomyces destructans is the only cause of illness, or whether other pathogens are involved. Nor are we certain of the various ways in which the disease spreads; as well as the role of humans in spreading fungus, scientists have pointed to the typically quite-high levels of bat-to-bat interaction. In autumn, the mating season brings together large numbers of males and females, while hibernation sees bats resting in large, tightly packed groups. Indeed, as things stand, scientists are still not even confident of how and why white nose syndrome kills the affected animals. One theory is that infection interrupts bats' hibernation, forcing them to use up precious energy reserves.
Whatever the answer, more work is needed if the world's bats are to be saved. In a recent review published in Conservation Biology, a team of scientists led by the University of California, Davis's Janet Foley argued for the creation of a "road map" to tackle the problem. "In the three years since its discovery, WNS has changed the focus of bat conservation in North America," they wrote. "A national response is required."