Fewer bats this Halloween: Scientists may be closing in on why

Bats are one of Halloween’s most enduring icons, but the mysterious creatures’ silhouettes against the moon are disappearing in the Northeast. A baffling illness has wiped out more than 75 percent of bat populations in the abandoned mines and ice-encased caves where they hibernate.

Now, researchers are reporting today in the online edition of the journal Science that they think they are closing in on a cause: A cold-loving soil fungus that seems to thrive on several species of hibernating bats during the winter months. The white fungus is so noticeable on the bats, researchers last year dubbed the sickness "white-nose syndrome."

Little brown bat with fungus on muzzle (Photo courtesy of Al Hicks, NY DEC)

Scientists don’t know if the fungus, from the genus geomyces, is a new species or some mutation of one that had already existed in the dark, damp caves and mines that bats are now entering to begin their long winter hibernation. But they do know that the fungus grows best between 41 and 50 degrees Fahrenheit -- and many bat hibernation spots often hit those temperatures.

"It grows slowly just like mold grows on your cottage cheese in the refrigerator," said David Blehert, director of diagnostic microbiology at the US Geological Survey National Wildlife Health Center in Wisconsin and the lead author on the report. But in hotter temperatures, he said, it grows far more slowly.

The Globe has had several stories about the bats at

http://www.boston.com/lifestyle/green/articles/2008/05/04/what_are_these_bats_telling_us_about_the_environment_we_live_in/

http://www.boston.com/news/science/articles/2008/07/28/wing_damage/

It’s still not clear exactly why the bats are dying -- the leading hypothesis is that the fungus bothers the bats so much they wake up to groom themselves during the exact time they need to be conserving energy to make it through the long winter. In Vermont, Massachusetts, New York and Connecticut, hundreds of skinny bats last winter were found weakly flying in the middle of the winter day -- possibly looking for food.

It’s unusual for a fungus to be a primary cause of death. Normally, they tend to be secondary infections -- so much so that scientists originally thought the bat fungus couldn’t be the main culprit. Yet if it is, Blehert said it may have parallels to chytridiomycosis, a lethal skin fungus that has caused worldwide amphibian declines.

All this is not good news for bats. The spores of the fungus seem to be able to withstand a large temperature range, meaning it could survive in caves to re-infect bats that survived it the previous year. And it could potentially survive on bats themselves to transmit to other bats as they mix together during the warmer months.

Scientists also don’t know how it got into the caves and mines -- or how to stop it from spreading.

"We can do all we want to attempt to control human movements, but we can’t control ... wild bats," Blehert said.