Bats use leaves as mirrors to find prey in the dark

Bats can find motionless insects on leaves in the dark only by echolocation. This was thought to be impossible, because insects are acoustically camouflaged by the loud echo of the leaves which confuses their echolocation system. Biologists from the Vrije Universiteit Amsterdam and the Smithsonian Tropical Research Institute in Panama have now discovered how bats overcome this problem.

08/06/2019 | 10:58 AM

Bats flood an area with sound waves and then use information from the returning echoes to navigate through the environment. Leaves reflect echolocation signals strongly, masking the weaker echoes from resting insects. So in the thick foliage of a tropical forest, echoes from the leaves may act as a natural cloaking mechanism for the insects, known as acoustic camouflage.

Acoustically camouflaged prey
New experiments show that by changing their approach angle, the echolocating leaf-nosed bats can use this sixth sense to find acoustically camouflaged prey. These new findings, published in Current Biology, have exciting implications for the evolution of predator-prey interactions.

Approach angle revealed
“For many years it was thought to be a sensory impossibility for bats to find silent, motionless prey resting on leaves by echolocation alone," says Inga Geipel, postdoc researcher at STRI. Geipel and Ralph Simon postdoc researcher at the Vrije Universiteit Amsterdam, discovered how the bats achieve the impossible. By combining evidence from experiments using an artificial bat head to create and measure ultrasound echoes, with evidence from high-speed video observations of bats as they approach prey, the importance of the approach angle was revealed.

More than 500 positions
To understand how bats overcome acoustic camouflage and seize their prey, Simon measured echoes with his biomimetic bat head and aimed sound waves at a leaf with and without an insect from more than 500 positions in order to create a full, three-dimensional representation of the echoes. At each position, they calculated the intensity of the echoes for five different frequencies of sound that represent the frequencies of a bat's call.

Like a mirror
vleermuis 1“When I analyzed the echoes it was very clear that there are certain positions where it will be easy to detect the insects” says Simon. “If bats would approach from these angles they would instantaneously detect the insect.” Leaves both with and without insects strongly reflect back the sound if it comes from straight ahead (i.e., from angles smaller than 30 degrees). When a bat approaches from these angles, it cannot find its prey as strong echoes from the leaves mask the echoes from the insect. If the sound originates from oblique angles greater than 30 degrees, the sound is reflected away from the source and leaves act like a mirror, just as a lake reflects the surrounding forest at dusk or dawn. The approach angle makes a resting insect detectable.

Between 42 and 78 degrees
Based on these experiments, Simon and Geipel predicted that bats should approach resting insects on leaves from angles between 42 and 78 degrees, the optimal angles for discerning whether a leaf has an insect on it or not. Next, Geipel recorded actual bats at Barro Colorado Island research station in Panama as they approached insects positioned on artificial leaves.
Using recordings from two high-speed cameras, she reconstructed the three-dimensional flight paths of the bats as they approached their prey and determined their positions. She discovered that, as predicted, almost 80 percent of the approach angles were within the range of angles that makes it possible for the bats to distinguish insect from leaf. "This study shows how we can achieve a better understanding of natural mechanisms, in particular bat echolocation, by simulating them with a technical setup” Simon says. “And understanding the acoustic world of a bat, may on the other hand lead to new applications like bat inspired sonar systems.”