Bats are effective sailors. Like many small submarines equipped with sonar, they deftly navigate dark forests and caves by listening to the echoes of their own voices. But how bats can tell which echo to follow when navigating a sea of overlapping and competing signals from myriad surfaces in their environment has been a mystery until now.
In a new study published In Proceedings of the Royal Society B, Researchers have provided evidence that bats find their way by listening to how their own movements alter sound.
Imagine you are at a party where hundreds or even thousands of people are talking at once; Mark Holdreid, a professor of sensory biology at the University of Bristol in England and author of the study, explains that it is difficult to identify any one speaker. This can be compared to the situation when a bat is dealing with an animal moving around a dense forest – a chaotic environment that can make echolocation difficult.
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To solve this problem, animals rely on the Doppler shift, or how the pitch of a sound changes as the bat travels.
“As the bat is moving, this Doppler shift, in this complex echo of thousands of reflectors, carries information,” says Holderied.
How the team came to that conclusion is an impressive and strange story. Holdreid and his colleagues observed wild pipistrelle bats using a device they dubbed the “bat accelerator.” The machine is basically an eight-meter tunnel of treadmills covered with plastic leaves — about 8,000 of them stapled together by hand, explains Athiya Haron, a medical engineering research associate at the University of Manchester in England and a study co-author.
The researchers theorized that if bats caught the Doppler effect, the direction in which the leaf treadmill was moving would affect how fast the animals flew.
When the treadmill moved in the direction of the bats’ flight, the creatures’ speed increased. However, when they saw leaves coming towards them, they slowed down. “We tricked them into thinking their speed was different,” says Holderied.
The results show that bats take the Doppler effect into account when flying and use it to control their speed.
Researchers already knew of some bat species that are so-called Doppler specialists, but pipistrelle bats are not among them, Holdreid says. The new findings indicate that the Doppler effect is used by bats that are not Doppler experts.
Haron says this bizarre experiment could help engineers improve navigation systems for drones or self-driving cars – something he has already begun to explore. “If this happens, it will benefit many navigation systems that fail in these types of cluttered environments,” she says.
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