(PhysOrg.com) -- It's something of an ecological murder mystery countless numbers of bats are turning up dead near wind farms. But what is killing them?
A new study from the University of Wisconsin-Madison links on-the-ground sleuthing and clinical diagnostic techniques to sketch a better picture of how the bats are dying.
UW-Madison forest and wildlife ecology professor David Drake and former master's student Steven Grodsky have conducted environmental assessments, funded by the renewable energy company Invenergy and Wisconsin Focus on Energy, of the Forward Wind Energy Center in southeastern Wisconsin.
They recently partnered with Melissa Behr and others at the UW-Madison School of Veterinary Medicine and Wisconsin Veterinary Diagnostic Laboratory to examine bat carcasses found near turbines at the site for clues to their demise.
The researchers had two primary suspects: blunt-force trauma from colliding with the turbine blades or poles, or barotrauma caused by flying through areas of different pressure created by spinning turbine blades.
Bats can easily navigate around stationary objects but the spinning turbines where the blade tip can be moving about 175 miles per hour pose a problem. With an echo-location range of about 60 feet, Drake says, "a bat would have roughly a quarter of a second to react to a turbine blade not very long at all."
And although bats are sometimes able to avoid a direct hit and fly between the blades, the dramatic air pressure change surrounding a blade can cause serious internal injuries, akin to the bends that affect a human diver who ascends too quickly.
"Bats' anatomical structure is not strong enough to absorb the pressure differential experienced," explains Drake. "As they hit that pressure gradient, it can cause their internal organs to explode."
A much-publicized study in 2008 used field observations of dead bats to suggest that barotrauma might be the primary culprit. The Wisconsin-led group used veterinary diagnostic techniques, including x-rays, tissue analysis, and gross necropsy, to look for more definite signs.
They identified a large number and type of injuries, including many that were not externally visible. Nearly 75 percent of the bats had broken bones, mainly in the wings, and the majority had sustained a mix of skeletal fractures and soft-tissue damage such as ruptured organs, internal bleeding, and hernias.
The researchers did not find specific patterns of injuries indicative of particular causes of death and concluded that both factors are at play. However, bats with few or no broken bones were more likely to be found closer to the turbines, suggesting that barotrauma felled these bats almost instantly.
"Barotrauma is a factor but it is not the clear-cut factor," Drake says. "There is certainly barotrauma going on, but there is definitely also blunt-force trauma from colliding with the turbine blade or possibly the monopole that holds the turbine up. Our results suggest bat deaths are the combination of both."
Roughly half of the bats examined also had middle and/or inner eardrum ruptures. Drake notes that such damage would not immediately be fatal but would disorient an animal, impair its ability to navigate and hunt, and likely hasten its demise. These non-instantaneous deaths may lead to an underestimation of the true extent of bat mortality near wind farms, he adds, since injured animals may be able to fly outside the search area before dying.
The issue is taking on greater urgency with the spread of white-nose syndrome, a deadly fungal disease that has decimated bat populations in the northeastern and eastern U.S. Without a better understanding of bat ecology, Drake says it's hard to predict the combined impacts of turbines and disease.
"We still don't know exactly why bats are being killed why the bats can't see such a large thing protruding from the landscape, or what is possibly attracting the bats," he says, "but now that we know direct causes of death we can start thinking about how to redesign turbine blades to have a smaller pressure differential or identify other cost-effective mitigation strategies that would minimize damage to bats."
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The study, "Investigating the causes of death for wind-turbine associated bat fatalities," is published in the October 2011 issue of the Journal of Mammalogy.