Jessica Marshall, Discovery News


Aug. 25, 2008 -- Researchers have found the cause behind mysterious bat deaths near wind turbines, in which many bat carcasses appeared uninjured.

The explanation to this puzzle is that the bats' lungs effectively blow up from the rapid pressure drop that occurs as air flows over the turbine blades.

"The idea had kind of been floating around, because people had noticed these bats with no injuries," said Erin Baerwald of the University of Calgary and lead author of a study about the finding in the journal Current Biology.

Researchers examined a large sample size of hoary and silver-haired bats found under wind turbines, performing necropsies on the bats within hours of their death.

The damage from rapidly expanding air in the lungs caused by the sudden drop in pressure was clear. Ninety percent of the bat deaths at the southern Alberta site involved internal hemorrhaging consistent with such damage, called barotrauma, while only 50 percent showed signs of collision with turbine blades.

For those overlapping cases, it may be that the bats flew through the pressure drop, suffered barotrauma, and then were struck by a blade. It is also possible that they were struck first, causing internal hemorrhaging.

But, Baerwald said, "When people were first starting to talk about the issue, it was 'bats running into the turbine blades.' We always said, 'No, bats don't run into things.' Bat's can detect and avoid all kinds of structures."

In fact, they are even better at detecting moving objects, Baerwald said.

"This kind of answers that mystery," she added. "It was something nobody could have predicted."

The bat fatalities appear to be a more significant problem than bird deaths from wind turbines in most locations. "Here we're picking up ten bats for every bird," Baerwald said.

"I can pick up nine different species of bird. I can pick up two species of bat," she added. "The impact on the populations is very different."

Whether these deaths are having a significant effect on the bat populations in Alberta or elsewhere is difficult to gauge because so little is known about the bats.

All species are susceptible to death by sudden change in air pressure, Baerwald said. "But the larger the animal is, the bigger the air pressure drop has to be. We know that four kilopascals [a unit of pressure] is enough to kill a rat. Bats are much smaller. We found that these wind turbines produce a five to 10 kilopascal drop."

Birds are less vulnerable to the drop, because they have rigid, tubular lungs, compared to the balloon-like structures of bat lungs, which are much like human lungs.

"It's one of those things we have speculated on for a long time," conservation scientist Edward Arnett of Bat Conservation International in Austin, Texas, told Discovery News.

"It's an important finding on the cause of the fatalities. They're not offered much room for error. If they avoid being struck at the last minute, they still may be killed by this rapid change in air pressure."

However, he added, "It may not lead us directly to any solution. Whether they're getting struck or they're dying from the barotrauma may or may not make any difference. We have to find ways to keep them away from the turbines."

"There are a lot of people testing different forms of mitigation," Baerwald said. "Right now the most promising one is to shut turbines down during slow wind speeds during the fall migration at night." These are the conditions when bats are most active.

Tests of this approach at her site in Alberta and elsewhere are promising, she said.