Sunday, September 17, 2017

Why Bats Crash Into Buildings

A new study suggests that smooth, vertical surfaces can trick the echolocation abilities of bats.CreditStefan Greif
Bat echolocation is a finely tuned sense. By emitting high-frequency calls and listening for returning echoes, bats can deftly navigate complex surroundings and precisely target moving prey in the dark. They can even discriminate differences of less than one millimeter in surface textures.
But this extraordinary capability is not foolproof. A study published Thursday in Science reveals a weak spot in bat echolocation: smooth, vertical surfaces such as the metal or glass plates on buildings can trick a bat into thinking it is flying in open air.
The findings may help explain why the creatures are often found dead or injured near buildings and other smooth structures, said Stefan Greif, an author of the study and a postdoctoral researcher affiliated with the Max Planck Institute for Ornithology in Germany and Tel Aviv University in Israel.
When an animal is fooled by a false environmental cue, it’s called a sensory trap. In a previous study, Dr. Greif showed that smooth metal or plastic plates, when laid on the ground, act as sensory traps for bats, which mistook the surfaces for water. In the process of setting up one of those experiments, he propped a plate vertically against a wall and noticed that bats intermittently crashed into it. He suspected he had accidentally stumbled upon another sensory trap.
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“I had to have a closer look,” he said.
In his latest study, which he led with Sándor Zsebők, also at the Max Planck Institute for Ornithology, Dr. Greif flew greater mouse-eared bats, a European species that often lives near humans, around a dark, rectangular flight tunnel. Near one corner of the tunnel, the researchers placed a metal plate either on the ground or against the wall. Of 21 bats, 19 hit the vertical plate at least once. In contrast, the bats never hit the horizontal plate or any other part of the tunnel.
A bat collides with a flat, smooth surface in a tunnel set up by researchers. Stefan Greif
Following up these lab findings, the scientists found in cursory field experiments that bats of two additional species — Schreiber’s bats and soprano pipistrelles — also crashed into vertical plates placed near cave exits or bat roosts.
When a bat approaches a smooth, vertical surface from an angle — as it would when turning a corner in a rectangular tunnel — its echolocating calls mostly reflect away from it. To understand this, try shining a flashlight at a mirror from an angle in the dark. You should see that light reflected onto the wall opposite the mirror, at roughly the same angle that separates you and the mirror.
It’s not until a bat gets very close to a flat, vertical surface that some of its calls end up hitting the plate at a 90-degree angle and bouncing right back. At this point, Dr. Greif and his collaborators noted, bats tended to change their echolocation patterns, shortening the time between calls, to collect more information. But it was often too late — out of 78 instances they observed of bats coming close to a vertical plate, 25 resulted in near misses while 53 resulted in crashes.
Brock Fenton, a bat expert and professor emeritus at Western University in Canada, said that he wasn’t entirely convinced of the study’s practical significance because bats use both echolocation and vision to navigate. Contrary to the popular expression, bats are not blind, he said, and one of the big questions in bat biology is how the animals respond if their “eyes tell them one thing and echolocation tells them something else.”
Dr. Greif acknowledged that how bats integrate echolocation and vision is a future avenue he hopes to explore. For now, however, he does see preliminary evidence that sensory traps can pose a problem to echolocating bats. In a long-term survey of birds hitting McCormick Place, a giant convention center in Chicago, for instance, researchers found at least 107 dead bats over a 30-year period.
“One of my main hopes for this publication is that people finally start paying a little closer attention to where they find injured bats,” he said, adding that the information could prove particularly important in preventing accidents along bat migratory routes.
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