Dark night for bats

New theories about what's wiping out huge populations of the tiny winged mammal point to pesticides and climate change.

Published August 1, 2008 11:45AM (EDT)

As dusk settles over the forest, the mosquitoes start swarming in force. Scott Darling, a biologist with the Vermont Fish and Wildlife Department, unfurls a net across a wide path. Not five minutes later, the first bat of the night lands in the net with a sudden thwoomp. The tiny winged creature bares its pointy teeth and begins to chirp, the angry staccato squeaks ringing out like Morse code.

Darling uses the dull point of a pencil to gently pry the net away from the entangled bat. Later, he will examine the bat for signs of disease, weigh it (7 grams, slightly more than a pair of pennies), tag it and set it free. Then he'll discard his latex gloves, slather hand sanitizer on his skin and disinfect his equipment, even dousing the pencil he used to free the bat from the net. This last bit -- the latex gloves, the disinfectant -- is still a new practice, a cautionary protocol courtesy of white-nose syndrome (WNS), a mysterious new illness causing bats in the Northeast to waste away as they hibernate.

Biologists named the syndrome for the unusual white fungus growing on the muzzles and bodies of many of the dead and dying bats. But most bat biologists believe the fungus is a symptom of WNS rather than the cause -- an opportunistic infection running amok in an already weakened bat. What's behind the deaths? Scientists don't know exactly, but they know the condition's deadly. Indeed, it looks as though WNS is to bats what colony collapse disorder is to bees, another baffling lethal syndrome.

"This is the worst crisis I've ever seen," says Merlin Tuttle, founder and president of Bat Conservation International. "I think anytime you have animals as ecologically essential, and as distantly related, as bees and bats dying en masse, it should send a canary-in-the-coal-mine signal."

Bat experts are working hard to decipher that signal. So far, they have more questions than answers. More than a few wildlife biologists have raised concerns that the bat deaths may stem from the way humans are treating our shared habitat. They are taking a close look at the impacts of pesticides, pollutants and possibly even climate change on the miniature mammals. "Bats are very good at living in high-stress environments," says Elizabeth Buckles, a wildlife pathologist at Cornell University College of Medicine. "But they've reached some kind of threshold where they can't adapt anymore."

Insect-eating bats fluttered around North America for millions of years before humans appeared on the scene. The winged mammals don't get the love that birds and bunnies do, but they fill a crucial environmental niche. A bat can eat half its weight in insects every night; if the female bat is lactating, it swallows twice that.

Bats have three favorite food groups: flies, moths and beetles. Mosquitoes (a member of the fly family) carry disease like West Nile virus, which has killed hundreds of species of birds in the United States and infected some humans, too. Many moths and beetles, and particularly their larvae, are serious agricultural pests that can destroy crops from apples to zucchinis.

"Bats are major ecological role players," says Tuttle. "They keep vast numbers of insects in check -- insects that cost farmers and foresters billions of dollars in losses every year."

The first signs that bats were in trouble showed up in New York two winters ago. The animals normally spend the winter months tucked inside caves and mines known as hibernacula. During routine surveys in 2007, staff from the New York State Department of Environmental Conservation discovered unusually large numbers of dead and dying bats lying in the snow outside the caves. The bats were thin, and many had an unusual white fungus growing on their faces. "It was very obvious to me that this was something new," says Al Hicks, the mammal specialist at the department who sent out the first warning cry.

By this winter, the mysterious malady had spread beyond New York to Vermont, Massachusetts, Connecticut and, most likely, Pennsylvania. Tens of thousands of bats perished. In the New York caves where WNS was first identified, 80 to 100 percent of the hibernating bats were lost. Of the six species that hibernate in colonies in the Northeast, WNS has struck five, including the federally endangered Indiana bat. The sixth species in the area, the big brown bat, is on the "suspicious" list.

It's all too easy to identify an afflicted cave, says Hicks. The ground outside the entrance is often littered with dead bats, and the white fungus is visible on many, though not all, of the animals. The bats are also extremely thin. They awaken and leave the caves months ahead of schedule, apparently because they lack the necessary fat stores to sustain them through their winter slumber. "The animals that have been recovered have been devoid of fat reserves," Hicks says. "They're starving to death."

Cracking the mystery is proving particularly challenging because scientists know so little about healthy bats. "We haven't known much about basic bat biology before this," Darling says. "That is science's little secret: We really don't know a lot of what people think we know, or what people think we should know."

Back in the woods, Darling identifies his squirming catch as a male little brown bat, the most common species in the area, and the hardest hit by WNS. "This bat looks pretty healthy," he says, sounding pleased and somewhat surprised. Just then, he gets a call on his cellphone. His colleagues at another site down the road report they've already snared 10 bats, so he sends me over to take a look.

When I arrive I meet up with Kristen Brissee, an enthusiastic wildlife technician in her mid-20s. She's removing bats from cotton storage bags strung up along the back of the Fish and Wildlife truck. She checks their body condition and attaches tiny metal I.D. tags to their wings. She gently spreads the wings of an Indiana bat and points out small white spots indicating scar tissue on the wing membrane. Another bat has several small holes in its wing.

Scientists suspect this wing damage is somehow connected to WNS. Some have theorized it may be caused by cold exposure when bats leave the caves prematurely and encounter icy winter temperatures. Others suggest the damage is caused by the fungus itself. Early studies turned up many different kinds of fungi growing on the bats, suggesting they were secondary infections thriving on already compromised bats. Recently, though, a common fungus species has been isolated from the sick bats. Ward Stone, a wildlife pathologist at New York's Department of Environmental Conservation, is growing the fungus in his lab. It seems to thrive in cold environments, like the chilly caves where bats spend the winters. "We think it's a brand-new species," he says.

Stone is working to put a name on the fungus and learn more about its biochemistry. The white stuff seems only to infect the surface of the bats' skin. So far, Stone hasn't found evidence that the fungus is infecting the bats internally. Nor has Buckles, who is also working to characterize the fungus at her Cornell pathology lab. Some investigators have suggested it may be an itchy irritant that prompts bats to wake up and groom themselves when they should instead be hanging around in a state of suspended animation. Yet that hypothesis doesn't fully explain the dramatic loss of fat reserves in the animals.

One possible explanation for the skinny bats is simply that they're entering hibernation in poor shape in the fall, without the fat they need to get through the long Northeastern winter. Perhaps there's a problem with their insect prey base, or an illness preventing them from putting on weight. "The other alternative is that they're coming in fat and happy, but something is happening while they're hibernating that causes them to burn up their fat reserves really quickly," Hicks says.

To Stone, that "something" is people. "We are involved -- oh, yes, we are involved," he says. Human-induced climate change is at the top of his suspect list.

Stone hypothesizes that dry summers over the past few years may have caused a decrease in the bats' insect prey, and that warm spells in recent winters may have triggered bats to awaken and burn up energy, which they couldn't replenish. "It would appear they were burning a lot more energy than they were taking in," he says. Climate change may have also allowed for the spread of the white-nose fungus, which may have been hiding under our noses for years, he says.

Despite his strong convictions, Stone can't explain how a warming climate would favor the spread of a cold-loving fungus, and he doesn't yet have evidence to directly link climate change to any of the symptoms of WNS. Stone has some respectable achievements under his belt; he was the first person to identify West Nile virus in the United States. But he's something of a contrarian, and hasn't convinced many other scientists of his theory. At a recent meeting of bat biologists, Darling says, the majority of attendees agreed that global warming was not a leading hypothesis.

That doesn't mean humans are off the hook. Many researchers say it's likely there is no WNS silver bullet, and that the syndrome may be caused by a thorny combination of factors. "Are they exposed to a new fungus? Is there a toxin depressing their immune system? Is there a problem in their food supply?" Buckles wonders. "Most diseases are multifactorial. We're not in a situation where we can rule anything out."

Buckles has looked for high levels of contaminants like lead and PCBs, and for chemicals from pesticides or other environmental toxins. The Northeast is downwind from the rest of the country's pollution, and all manner of agricultural pesticides are introduced into the landscape each year. Again, though, no obvious suspects have emerged. However, a pair of researchers in the Midwest recently uncovered intriguing data that hints pesticides may be involved.

John Whitaker is a biologist at the Center for North American Bat Research and Conservation at Indiana State University. Several years ago, he and collaborator Kathleen Dannelly, a microbiologist at ISU, discovered an enzyme called chitinase in the guts of bats. This enzyme breaks down chitin, the major component of insect exoskeletons. But mammals weren't known to produce chitinase. Digging deeper, the pair discovered a garden of chitinase-producing bacteria in the bats' intestines. The scientists concluded that the beneficial bacteria were helping bats draw energy from otherwise indigestible insect parts.

Whitaker dissected some hibernating bats. He found no fresh food in their stomachs -- not surprising in the middle of winter. But he did find pieces of chitin, presumably left over from the bats' summer meals. Whitaker and Dannelly believe the chitin pieces stored in the bats' guts are slowly digested over the winter, giving the bats extra energy to get through the season. So when they heard about a new syndrome causing bats to waste away while they hibernated, they were eager to take a look.

Whitaker and Dannelly examined 12 little brown bats from New York, all found dead in caves where WNS was rampant. Inside the bats, they found plenty of chitin. "We could see bug legs and wings," says Dannelly. But the chitinase-producing bacteria were harder to come by. She took multiple samples from each of the bats, up and down the intestinal tracts. She found extremely low numbers of the beneficial bacteria in one bat. In the remaining 11, she found none at all.

Dannelly's hunch is that a toxin may be killing off the symbiotic bacteria in the bats' intestines. "Maybe they're using a new pesticide [in the Northeast] and it's becoming more widespread," she suggests. Theoretically, bats could take in traces of a pesticide with their food or drinking water. If the chemical wiped out the beneficial bacteria, the bats couldn't break down the chitin in insect parts. It's the energy from that chitin, Whitaker believes, that may give bats the boost they need to survive the winter.

According to Dannelly, at least one relatively new pesticide is known to be toxic to chitinase-producing bacteria. "So far nobody has been able to tell me if it's used in the New York area," she says. She declines to give pesticide names, since she doesn't have evidence implicating any specific chemical. And she and Whitaker admit their data isn't perfect. They examined only 12 sick bats, and they weren't able to examine healthy bats from the Northeast to use as a control. Instead, they compared the gut bacteria of sick bats with those of healthy little brown bats from Indiana. Their findings are "a powerful clue" that the disease may be linked to a loss of beneficial bacteria, Whitaker says. But it's not an answer. "We haven't proved it yet."

He and Dannelly are hoping to get their hands on more bats to boost their sample size. Meanwhile, they're attempting to calculate exactly how big a role chitin plays in the bats' total energy needs, in both summer and winter. Whitaker adds, with slightly detectable disappointment, that his idea hasn't gotten much attention from his colleagues. The theory "hasn't risen to the top of the pile," admits Hicks, who has emerged as the de facto leader of the WNS investigation. Then again, he says, "nothing much has. We're fishing now. Anything that shows potential, we'll pursue."

While lab scientists such as Buckles and Stone continue to pursue the fungal clues, field biologists like Darling are keeping tabs on the health of the bats, hoping to have a better sense of the animals' condition when they enter hibernation this fall. By Darling's count, more than 25 agencies and institutions are contributing to the effort.

They may not all see eye to eye on which angles to pursue, but nearly everyone agrees the syndrome could be disastrous to the bat population. Bats can live up to 30 years, and females give birth to just one pup per year. At that rate, it could take a very long time for a broken bat population to recover. The animals migrate hundreds of miles between hibernacula and summer foraging grounds, mingling with bats of other species and from many states. In its first year, WNS spread between 80 and 130 miles from the apparent epicenter in Albany, N.Y. "I'm afraid it could spread right across the country," says Hicks.

Darling worries that caves that appeared healthy last winter may have been in the early stages of the syndrome. "It may progress in such a way that it takes a couple of years for sites to be maximally affected," he says. If that's true, "we might be facing a really traumatic winter for the bats."

On the evening I spend bat-trapping with Darling, his primary mission is to catch a female Indiana bat and fit her with a radio transmitter. If all goes as planned, the transmitting bat will lead him and his team to the maternity roost, a tree where a mass of mother bats and their pups sleep off the daylight hours. If he can find the tree, he can better estimate the size of the endangered bats' reproductive population.

On this night, the team is energized by success. Two female Indiana bats land in the net within the first two hours. Both are lactating (a fact that wildlife technician Brissee establishes by squeezing the tiny bats' even tinier teats to look for milk), and will likely be heading to the maternity roost as day breaks. The biologists carefully glue the minuscule transmitters to the animals' backs and, one at a time, set them free.

It's just after midnight and the cloudless Vermont sky is overflowing with stars. We watch the last bat fly off, beeps from the transmitter antenna growing fainter as the female flutters away into the darkness. It's a magical moment, and I want to be optimistic for this bat and its kin. But white-nose syndrome hasn't offered much reason for optimism. "It could be years before we have an answer," Darling had told me earlier, as he trudged through the tall meadow grass with a squeaking bat in his hand. "I'm not sure the bats have that long to wait."

By Kirsten Weir

Kirsten Weir is a science writer who lives near Portland, Maine.

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