People near Albany, N.Y., began noticing the strange bat behavior at least two years ago: Droves of the normally nocturnal mammals were seen flying around on brisk winter days when they should have been hibernating in caves for the season. The state's Department of Environmental Conservation (DEC) teamed up with the U.S. Geological Survey (USGS) to investigate and made an alarming discovery: Bat populations throughout northeastern New York State, Connecticut, Maine and Vermont had thinned by as much as 97 percent in area bat caves and emaciated survivors were found hanging near cave entrances where it is typically too cold for them to stay the entire winter. The only clue to the mysterious phenom was a white, powdery organism on the muzzles, ears and wings of the dead and dying bats.

Scientists have since linked the deaths of more than 100,000 of the smaller species of brown bats, northern bats, tricolored bats, Indiana bats and small-footed Myotis, along with larger brown bats in the U.S. Northeast to a condition they dubbed "white-nose syndrome." These researchers suspect that, since the winter of 2006, the ailment may have contributed to a steep decline in the bat populations at many caves in the affected states; in the most extreme case, about 1,750 of 1,800 of the flying mammals were found dead. Bat declines at many surveyed hibernation caves exceeded 75 percent.

"If you apply this across [the northeastern U.S.] there could be 200,000 dead bats and possibly even more," says David Blehert, a USGS microbiologist and lead author of a paper on the syndrome published today in Science.

USGS scientists are working with New York State environmental and health officials to pinpoint the exact cause and consequences of the fatalities. A breakthrough came in April when Blehert identified the white organism on the critters' noses as a type of geomyces fungus, one of a group of organisms that live in soil, water and air and reproduce at refrigerator temperatures of 39 degrees Fahrenheit (four degrees Celsius), the temp in most bat caves.

"When bats are torpid [in a hibernating state], they drop their temperatures down to the ambient temperatures of the caves," Blehert says. This makes the dormant bats susceptible to infection by this fungus.

But researchers are in the dark about the source of geomyces. They don't know if its spores were carried to the bat caves by animals or the wind or if it was in these caves all along and recently spread to spots where the bats hibernate.

It is also unclear whether the fungus is killing the bats or is a contributing factor in their deaths. Most of the victims were also rail-thin and some were found outside of their caves, indicating they may have starved to death after an apparently futile attempt to find food (insects, primarily) in the winter.

"Fungi are opportunistic pathogens," Blehert says, "they don't usually attack and kill otherwise healthy animals."

One thing is certain, Blehert says, "Before the identification of white-nose syndrome, mass mortality events in bats as a result of disease were very rare."

Blehert and his colleagues are now attempting to determine whether the dead bats went into hibernation emaciated or filled with enough food to sustain them throughout the winter. "If they're entering with less than a full tank of gas," he notes, "we have to consider whether the insect populations that they feed on are down." Bats are meant to remain in a torpid state throughout the hibernation period, rousing only every two weeks or so to groom or drink water.

Another possibility, he says, is that the fungus killed them indirectly by causing so much skin irritation it prevented them from getting enough sleep. The longer they are awake during hibernation, the more fat they burn, which gives them less time to sustain their bodies without refueling.

Thomas Kunz, a biology professor and director Boston University's Center for Ecology and Conservation Biology, is also studying the mysterious bat deaths. "The mortality is unprecedented in my experience," he says, "and I've been working with bats for 40 years."

Blehert's work is groundbreaking, Kunz says, in that it has provided researchers with an understanding of the fungus, although the syndrome is most likely a secondary effect of some other underlying cause for the deaths. "This is serious in the sense that we're dealing with an unknown," he says, but he does not believe that the fungus itself is the pathogen.

Kunz and his team are approaching the mystery from three different angles: The first is to study the body weights of hibernating bats in different geographic areas by collecting samples of the creatures from three caves in the affected areas and from three caves in Ohio and Pennsylvania, where white-nose syndrome does not appear to have struck. Similar to Blehert's approach, this will inform Kunz and his team of whether the bats in the areas hit by the syndrome are beginning hibernation with the right amount of stored fat. If not, this might signal that pesticides are diminishing local insect populations, possibly choking off a primary food source for the bats.

The second angle is to determine whether the animals are storing the right type of fat (unsaturated fatty acids obtained by eating insects) for their dormancy, Kunz says. A lack of unsaturated fatty acids could again lead the scientists back to suspect declining local insect populations from insecticide use. A third area to investigate is whether the bats' immune systems are being suppressed for some reason, making them more susceptible to fungal infection. "There's no smoking gun at this point," Kunz says, but he and his colleagues are collecting bat samples at this time and hope to have some results by December.

In addition to the role bats play in vampire lore, these creatures of the night are indispensable to insect control, plant pollination and seed dissemination. White-nose syndrome's impact on the bat community is difficult to determine, Blehert says, because of the difficulty of keeping accurate population counts. (They tend to spend a good portion of their lives tucked away in caves inaccessible to people.)

Researchers have been able to count as many as 500,000 hibernating bats throughout the states were white-nose syndrome has been found, but Blehert says there are probably more than that. He likens the demise of the bats to the disappearance of amphibians worldwide over the past three decades that was ultimately traced to a lethal fungal skin infection—chytridiomycosis—which has wiped out entire populations

One possible bright spot is that some of the lesions on dead bats that Blehert and his colleagues examined had begun to heal before the bats died, which indicates that the bats are capable of fighting the infection to some extent. The researchers plan to spend this winter studying  the effect of this fungus on healthy bats in the lab. Blehert says he will be surprised if the fungus alone was the sole culprit behind the plummeting bat population. "I'm not sure a fungus," he says, "can kill an otherwise healthy animal."