Leaf-nosed bats can locate even small prey with echolocation by exploiting an "acoustic mirror" effect, according to a recent paper in Current Biology. If the bat approaches an insect on a leaf from an optimal angle, the leaves act as a mirror, reflecting sound away from the source. The research could have important implications for studying predator-prey interactions and for the field of sensory ecology.

It's common knowledge that bats hunt and navigate in the dark primarily by emitting ultrasonic pulses and using the returning echoes to determine the location, speed, and distance of nearby objects or prey (active echolocation). But different species of bat can use echolocation in slightly different ways, including passive echolocation strategies. The pallid bat, for instance, might use active echolocation for navigation but a passive approach when it hunts. It has two pairs of ears (internal and external), the better to pick up any noise generated by insects. But what about insects that don't make any noise, like the dragonfly?

Co-author Inge Geipel, a postdoc with the Smithsonian Tropical Research Institute (STRI), first became interested in the issue while working on her PhD at the Institute for Advanced Study in Berlin, Germany. Her thesis advisor, Elizabeth Kalko, had found dragonfly wings in leaf-nosed bat roosts—a surprising find, since dragonflies are diurnal, meaning they don't fly at night, settling in on vegetation instead. They don't have ears, so they can't hear hunting bats, nor do they produce sounds as a means of communication. Most bat scientists assumed dragonflies would be too small for the bats to find purely via echolocation.

"The weaker echoes of smaller objects, like insects, would be masked by the stronger background echoes," said Geipel. So how would the bats have found the dragonflies? Because they do find them, as Geipel's initial experiments with bats in a fly cage revealed. She knew that trawling bats use an acoustic mirror effect—a means of reflecting and focusing sound waves—to hunt over large bodies of water, and thought the leaf-nosed bats might use a similar method, just over a smaller surface area (individual leaves).

To find out, she built a "robo-bat" biosensor: essentially just a speaker to reproduce the five primary ultrasonic frequencies of bat calls, with a microphone right next to it to mimic the bat's ears and record the returning echoes. She moved the robo-bat around a synthetic leaf, both with and without a dragonfly on it, and then measured the echoes from about 500 different positions. From that, she was able to calculate the optimal angle of approach that produced the loudest echoes—the better to locate a sleeping insect on a leaf.

That turned out to be angles greater than 30 degrees, and optimally between 42 and 78 degrees. A bat approaching a leaf from a smaller angle would not be able to locate the dragonflies, since stronger echoes from the leaves would mask those coming from the insect.

"Imagine you have a torch, like a spotlight, and you're standing in front of a mirror," said Geipel. "If you shine it at the mirror directly, you will blind yourself. But if you're shining the light from an oblique angle, you can reflect the light to a different spot. That's basically what the bats are doing."

"We're still finding out just how sophisticated the echolocation system of these bats really is."

Next, Geipel needed to test actual bats in the field, so she went to STRI's Barro Colorado Island research station in Panama, which has a large fly cage in the heart of a tropical forest. She had to capture the bats herself, usually near their roosts, since they can be quite elusive in the air. She placed the bats in a smaller, custom-made fly cage within STRI's larger one. After giving the bats one night to acclimate, she started placing freshly dead insects on the artificial leaves in the fly cage—and yes, she had to catch those, too.

Then Geipel used high-speed cameras to capture the bats' flight paths and used that footage to reconstruct their positions as they approached their prey. It was tedious work. Bats eat a lot of insects, considering the bats weigh just six grams (less than an ounce), but they tend to eat one and then take a short nap before hunting for their next snack.

"My field work was basically me sitting in the fly cage, watching sleeping bats, waiting for those few seconds when they were actively searching for prey," said Geipel. But her patience and stamina paid off in the end. As she had predicted, nearly 80% of the bats' approach angles fell within the optimal range.

"I find it super fascinating to imagine perceiving the world through sound," said Geipel. "How do bats see the world? How do they hear their world? We're still finding out just how sophisticated the echolocation system of these bats really is."

DOI: Current Biology, 2019. 10.1016/j.cub.2019.06.076 (About DOIs).