Photograph by Wendy Shattil and Bob Rozinski, Photolibrary/Getty Images

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A little brown bat (file photo).

Photograph by Wendy Shattil and Bob Rozinski, Photolibrary/Getty Images

Bats Crash More When They Rely on Vision

Mysterious behavior may be influenced by hormones, study says.

Being blind as a bat apparently has its benefits: Wild bats that use their vision to fly short distances are more likely to crash into objects, new research says.

Bats can navigate both visually and acoustically, by sending out sound waves and listening for echoes bouncing off objects—including prey. Bat vision is generally known to be sharpest in dim light, and to get worse the brighter it gets.

For the new study, scientists set up an obstacle course near an abandoned mine in Ontario, Canada, where little brown bats often gather.

The team manipulated three types of light conditions—dark, dim, and bright—and observed how little brown bats flying through the course behaved. The results showed that bats primarily relied on their vision to navigate the well-illuminated course—even though their reliance on vision made them more prone to crashing.

In the obstacle course, the team used fabrics of three different visibilities—a clear fabric, an opaque fabric, and a reflective fabric. If the bats were mostly using their sonar, they should have detected all three. But the bats did not sense some fabrics—such as the clear one—suggesting the animals were depending more on their vision, the scientists noted.

The study is among the first experiments to confirm such behavior in wild bats, according to study co-author Dara Orbach, formerly a graduate student at the University of Western Ontario in Canada. (See flying-bat pictures in National Geographic magazine.)

Past experiments have shown that blindfolded, captive Indiana bats ran into windows less often than bats that could see, and that little brown bats flying through a lab obstacle course crashed more often when the lights were turned up.

Hormone Switch Changes Crash Rates?

It's unknown why bats use their vision to their detriment. But the research also turned up a tantalizing clue: Midway through the study, when the bats' hormones shifted, so did their the crash stats.

"That was the really unexpected part," Orbach said. "We know there are two phases [of bats' preparation for hibernation]. During the first phase of swarming, during the month of August, they're flying around to different hibernation sites. And then there's this distinct day—at least at our field site—[when] there's a switchover."

After that turning point, the bats changed their eating habits, became sleepier during the day—like a temporary hibernation—and began "promiscuous" mating, Orbach said.

What's more, the mammals' behavior reversed: They began crashing more in the dark than in the light.

The switch between bats' behavior and the collision rates match up nearly perfectly.

"We don't know for sure, but our suggestion is that the way bats are using vision could correspond to their different needs" at different times, Orbach said.

"There seems to be this relationship that corresponds to hormonal or physiological changes in the bat."

The bat-vision study appeared November 9 in the journal PLoS ONE.