Bats bounce sound off leaves to find hidden prey

Most insects use camouflage to hide in plain sight from predators.

But an international research team found that some bats thwart this defense by finding insects on leaves or branches using sound alone. The discovery was published in the journal Current Biology.

University of Cincinnati biologist Dieter Vanderelst helped unlock the superpowers of the common big-eared bat, a tiny species found across South America, including a jungle island outside the Panama Canal. Barro Colorado Island is off-limits to visitors but serves as a research station for biologists and conservationists.

Researchers captured wild bats from a known day roost in a storm drain. They set up high-speed cameras and an array of audio sensors in a flight cage to observe how the little predators can find insects hidden among the leaves in total darkness.

An infrared camera captures a big-eared bat scanning a leaf in complete darkness to find a perched dragonfly. The leaf is studded with acoustical sensors. Video/ChiRoPing Consortium

The bats have a leaf-shaped appendage on their nose that helps them aim and focus their high-frequency chirps to scan their complex environment. And true to their name, big-eared bats have big ears in proportion to their bodies to capture the echo.

“They’re special in a sense because they’ve pushed their adaptations even farther,” he said. “They have very directional hearing. Their ears are huge. And they use very high frequency sounds compared to other bats. That allows them to have good time resolution.”

Vanderelst is an assistant professor of biology, psychology and electrical and mechanical engineering in UC’s McMicken College of Arts and Sciences and its College of Engineering and Applied Science. For some of his projects, he has taken cues from bats to improve autonomous drone navigation.

Researchers conducted experiments in a concrete bunker in the rainforest to block out ambient light. They taped a dead dragonfly to a leaf dotted with 20 microphones in a tiny bat flight cage. Then they used high-speed cameras to capture the graceful and precise navigational aerobatics of the bats as they hunted prey.

Field research in the tropics poses unique challenges. High humidity fogs camera lenses. The extreme isolation of the island meant long waits to repair or replace equipment. Four of five researchers sat for long periods in a cramped, hot and foul-smelling room shared by lots of spiders.

“It’s a nightmare scenario. You have to be a special kind of person to want to do that,” he said.

Big-eared bats are unusual among bats, said Inga Geipel, the study’s lead author and a fellow with the Smithsonian Tropical Research Institute. They specialize in hunting comparatively larger prey such as dragonflies and lizards. And they bring insects back to their babies for as long as five months after their pups are weaned

“I have been working with this species for over 12 years now and they surprise me over and over again,” Geipel said.

Geipel first observed the bat’s amazing echolocation in a 2013 study. For the latest paper, researchers tried to understand how they manage this feat.

“Previously it was thought to be a sensory impossibility to find silent and motionless insects resting on leaves just by using echolocation as the weaker prey echoes would be masked by the stronger echoes of the leaves,” Geipel said.

The little bats can hover like a hummingbird as they scan their environment. High-speed video revealed how the bats approach the target leaf from below and gradually ascend like a drone rising over a tree. They had no trouble finding the meal, plucking the tasty insect from the leaf and carrying it off to devour.

The high-frequency bat chirps strike the leaf’s surface at different angles. The pulses deflect off the leaf and bounce off the unwitting prey, reflecting back an echo to the bats that something besides the leaf is there. Scientists call this “specular reflection.”

“We found that the bat approached the insects in a stereotypical way, coming up from the ground,” he said. “And it uses the mirror effect. When you call against a flat surface, most of the echo bounces away from you. But if there is an insect there, it creates a reflector and you start to get an echo back. It’s an adaptive behavior to separate the background, which is conspicuous, from the prey.”

Researchers replicated the bat’s hunting strategy using sensors and artificial sonar to measure the energy returning from leaves at different angles. Indeed, the tests showed a discernible change in echo when an insect was present on the leaf.

Geipel said the bat’s refined echolocation even can help them distinguish types of prey. Experiments showed how the bats could distinguish between real dragonflies and dragonfly-shaped dummies made of paper and aluminum, she said.

The big-eared bat’s extraordinary echolocation opens up new hunting opportunities for them, Vanderelst said.

“It allows them to exploit a completely different niche,” Vanderelst said. “Most bats like to stay away from vegetation while hunting. Some bats will wait for the flying insect to take off and become conspicuous. Others will stop echolocating and listen for the sounds generated by the insects.”

Bats and their prey are in a never-ending arms race. Some moths are covered in sound-dampening scales to reduce their acoustic footprint like a stealth fighter. Others have evolved to freeze in midair and drop like a stone when they hear a bat nearby. Still others can emit their own high-frequency sounds to disrupt bat calls, effectively jamming the sonar. And bats have responded, evolutionarily speaking, by calling more quietly to detect moths without alerting them to their presence, Vanderelst said.

Many plants have developed a relationship with nectar-eating bats who serve as their primary pollinators. Scientists long puzzled over why some flowers had unusual bowl-shaped leaves next to their blossoms until they realized they provided a sound-reflecting signal to help bats find the delicious blooms in the dark. 

Yes, a bat signal.

“It’s a wonder that biologists missed that for so long,” Vanderelst said. “Now it’s obvious what the leaf is for.”

Researchers still don’t know just how good bats are at identifying individual prey by sound. Vanderelst said he isn’t convinced the bats have refined their abilities to the point where they can tell a tasty insect from a toxic one, for example.

“Some people think bats build a highly detailed image of what the insect looks like on the leaf,” he said. “But I think what is happening is much simpler. It sees if there’s something there to eat. If there is, I’ll chew it. And if it doesn’t taste good, I’ll spit it out.”

The experiments took place in a simple environment with few navigational obstacles or distractions. Vanderelst said he would like to know more about how bats hunt prey in a vastly more complex rainforest.

And the research opens up more questions about the secret abilities of animals and how they might translate to engineering.

“I’ve always been interested in what animals perceive. What does a spider see? What does a dog smell? If I were a snake, how would the world be different?” he said. “You can look at it from a technical or biological point of view.”

Featured image at top: A nectar-drinking bat uses echolocation to find a flower. Species such as the common big-eared bat can use sound to tell insects from leaves. Photo/Zdeněk Macháček/Unsplash