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Bats Change Signals When Going for the Kill

Recordings of bats chasing prey through a field of microphones have explained why the insect-hunters change their calls when swooping in for the kill.

Lower echolocation frequencies provide a wider field of view, making it harder for prey to vanish from the bats’ sound-assisted sight.

“A lot of insects can hear ultrasound. If it’s really close to them, they do evasive maneuvers. Fold their wings up, go into power dives,” said biologist Lasse Jakobsen of the University of Southern Denmark. “We thought this could be a way for bats to counteract this.”

In the Daubenton’s Bat species tested by Jakobsen in a July 19 Proceedings of the National Academy of Sciences study, call frequency drops by a full octave as bats approach insects. That’s typical of the roughly 500 bat species who use echolocation to snatch insects on the wing, but scientists haven’t known why.

“People thought they couldn’t maintain the high frequencies, or that it had something to with bandwidth,” Jakobsen said.

Jakobsen and USD zoologist Annemarie Surlykke used an array of 12 suspended microphones to record captive bats’ echolocation calls while hunting. By comparing the intensity of calls at each microphone in relation to a bat’s location, they calculated the range covered at each frequency.

High frequencies used early in the hunt are like a flashlight, casting a relatively narrow beam. Lower frequencies are a light bulb, illuminating a wide circle. When bats get close, bugs have nowhere to hide.

Jakobsen next plans to study whether the shape of echolocation beams remain the same between species of different sizes and habits. “Beam shape plays a large role in how they perceive the world,” he said.

Image: Graphical representation of bat echolocation field during search (blue) and approach (red) phases of hunting./PNAS.

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Citation: “Vespertilionid bats control the width of their biosonar sound beam dynamically during prey pursuit.” By Lasse Jakobsen, Annemarie Surlykke. Proceedings of the National Academy of Sciences, Vol. 107 No. 29, July 20, 2010.
