Busy bats crash into windows due to 'acoustic illusions'

September 11, 2017, 9:52 a.m.
Mexican free-tailed bats
Photo: Dan Pancamo/Flickr

Windows can be notoriously dangerous for birds, which often mistake glass facades for open airspace. This optical illusion leads to widespread collisions with windows, killing hundreds of millions of birds every year in the United States alone.

But according to a new study, windows can pose a similar threat to bats. The flying mammals may not be visually confused, since many rely on echolocation when flying after dark, yet they can apparently still misinterpret windows when they're in a hurry.

To navigate by sound, bats make noises and then listen for the echo, using auditory details to build a mental map of their surroundings. This helps them swoop through trees and grab insects in midair, but despite bats' acrobatic skill, many of their ancient habitats now feature unusually smooth surfaces (like glass or metal) that can be hard to identify by echolocation.

A smooth, vertical surface creates "acoustic illusions," the study's authors explain, by reflecting echolocation calls away from a bat until just before collision. Without hearing its calls echo back, the bat may think the path ahead is clear. And if it isn't paying close attention — perhaps because it's busy hunting — it may not have time to veer out of the way before it crashes.

The study's authors discovered this effect by accident, while working on a different study in 2010. In that study, researchers Stefan Greif and Björn Siemers — sensory ecologists at the Max Planck Institute for Ornithology in Germany — showed that bats tend to perceive smooth, horizontal surfaces as water. And that makes sense: In a natural landscape, lakes and rivers are the only smooth, spatially extended surfaces a bat is likely to encounter. This association seems to be instinctive, the researchers concluded, since bats didn't stop trying to drink from a metal plate on the floor, even after several failed attempts.

During those earlier experiments, the researchers also noticed bats crashing into some metal plates they had left propped upright. Their new study investigated that quirk by encouraging greater mouse-eared bats to fly in a tunnel known as a flight room. When a smooth metal plate was placed against the wall, 19 of 21 bats crashed into it at least once within the first 15 minutes. No bats tried to drink from the vertical plate, but when it was moved to the ground, 13 bats tried to drink from it and none crashed.

bat drinking water in flight Bats drink water in flight, swooping down to sip from the surface without trying to land. (Photo: Dennis W. Donohue/Shutterstock)

No bats were harmed in this study, the researchers note, thanks to limited space in front of the panel that prevented them from flying at high speeds. In less controlled conditions, however, collisions like these could potentially cause injury or death.

The researchers recorded their experiments with infrared cameras and microphones, revealing three basic categories of bat crashes: "near collision," in which bats barely avoided the plate; "collision with maneuver," in which they collided despite last-minute evasive maneuvers; and "collision without maneuver," in which they collided with no noticeable attempt to dodge.

When a bat first approached the plate from a distance, its calls were reflected away by the smooth surface, creating a false impression of empty space. Just before impact, however, a strong, perpendicular echo finally reflected back to the bat, giving it one last chance to swerve. The bats that hit the plate produced fewer calls, and spent less time directly in front of the plate.

"So when bats had enough time and information to process the situation, they were able to recognize the perpendicular echoes and reveal the open flyway as an acoustic illusion," according to a news release from the Max Planck Institute. The researchers also conducted similar experiments in the wild, where they observed bats from three different species colliding with the panel.

"They think it's an opening," Greif tells New Scientist. This effect is less well-known than birds' window woes, although Greif cites anecdotal reports of people finding dead or injured bats near buildings. It's unclear how often this happens, but with many North American bat populations already plummeting due to white-nose syndrome and habitat loss, any additional threat could be a big deal. Bats' trouble with smooth surfaces might also help explain why they fall victim to wind turbines in some places, Greif adds, although the speed of turbine blades may play a larger role in that phenomenon.

Bats are worth saving for more than just ethical or scientific reasons. They perform key ecological services by eating insects, for example, many of which destroy crops or transmit disease. Just by keeping crop pests in check, bats save U.S. corn farmers an estimated $1 billion every year. Their value to U.S. agriculture ranges anywhere from $3.7 billion to $53 billion per year overall.

Birds and bats aren't stupid, but as the study's authors explain, humans are altering their habitats too quickly for many to adapt.

"Human-generated structures now dominate much of the planet, but they have existed for but a blink of an eye from an evolutionary perspective," they write in the journal Science. And because animals' sensory systems "evolved to navigate natural environments," they may not always be reliable in habitats altered by humans. "With millions upon millions of smooth vertical surfaces in our world today, such misperceptions could have considerable negative impacts on bat survival."

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