How do bats tell insects from leaves? Ask a robot

Led by the University of Cincinnati, engineers programmed an autonomous robot to find a dragonfly hiding on leaves using only electronics that mimic a bat’s echolocation. The robot uses a nimble, jointed arm fitted with stereo microphones that, like a bat, can detect the reflective echoes of pulses it bounces off surfaces as it rolls across a track.

Most bats use echolocation to find food by emitting high-frequency chirps to catch mosquitoes, moths and other flying insects in midair.

But big-eared bats have the ability, extremely rare among bats, to find insects hiding on leaves. Vanderelst said the bats take advantage of the specular reflection effect, in which noise bounces off a surface at the same angle that the sound strikes it.

Vanderelst and his research partners previously demonstrated how the bats can find hiding prey on leaves with sniper-like accuracy in total darkness.

Vanderelst witnessed this behavior firsthand at the Smithsonian Tropical Research Institute in Panama where he and his research partners erected a 15-foot-square flight cage and used a suite of night-vision cameras to capture the hunting techniques of wild big-eared bats.

Researchers captured the bats in the late afternoon from known roosts where they spend the daylight hours and released them into a flight cage.

“At first they panic and flutter around, but then they settle down and feed,” Vanderelst said.

The foliage in the flight cage represents a novel landscape that the bats have to navigate to find food, Vanderelst said. And the video demonstrates how the bats take precious little time identifying a target, in this case dragonflies.

“For a bat, dragonflies are like children’s squishy squeeze snacks. They bite the dragonfly’s body and squeeze out the insides,” he said, adding, “It’s not very pretty.”

In the morning after capturing hours of video footage, researchers released the bats back to their daytime roosts.

Big-eared bats use echolocation sparingly, he said.

“They’re super quiet. You don’t pick up a lot of their calls on microphones,” he said. “They’re called whispering bats because they’re so quiet compared to other bats.”

This probably provides a hunting advantage since some moths and other insects have evolved to hear the frequencies of echolocating bats to help evade them.

“This study nicely shows how you can use robots as physical simulation tools to study the mechanisms behind the behavior of bats,” said co-author Herbert Peremans, an engineering professor at the University of Antwerp.

“It might get us a small step closer to understanding how bats use their sophisticated echolocation system to ‘see’ their world through sound,” added study co-author Inga Geipel, a behavioral ecologist also at the University of Antwerp.

Peremans has partnered with Vanderelst and other researchers on many studies of bats in his career. They never fail to amaze him, he said.

Previously, Vanderelst and Peremans discovered that huge colonies of bats can share airspace safely without collisions using their echolocating calls in the raucous, crowded confines of a cave opening. They likened this to cooperative signal processing, allowing sometimes millions of bats to come and go in tight spaces without deadly collisions.

“Oh, I have always been a great believer in the remarkable abilities of bats to use echolocation to survive under challenging conditions,” Peremans said.

And their unusual superpowers could hold the key to unlocking new abilities for autonomous systems, he said.

“I believe there is still a lot more to find out about bat sonar,” he said. “In particular, how the combination of signal processing, environmental constraints and bat behavior allows the bat to efficiently deal with the difficult perceptual challenges it is confronted with will remain a treasure trove for engineers for many years to come.”

Featured image at top: UC Associate Professor Dieter Vanderelst used robotics to explore how common big-eared bats find insects hiding in thick foliage. Illustration/Margaret Weiner/UC