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When it’s too noisy for bats to listen for their prey, they use echolocation to hunt instead

When it’s too noisy for bats to listen for their prey, they use echolocation to hunt instead
A fringe-lipped bat forages above the forest floor. (Alex Lang)

The fringe-lipped bat hunts by ear.

The winged predator, found in South and Central America, eavesdrops on the male tungara frog's mating call and decides on the moment to strike. When the time is right, the bat flies from its perch, using echolocation to hone in on its prey and strike.

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Many animals, like bats and owls, rely on sound to hunt, and a great many more make sounds to communicate. But as man-made noise pollution floods ecosystems, scientists are wondering how wildlife will adapt. What can a bat do when road noises mask the mating call of that tasty frog?

Dylan Gomes, then a researcher at the Smithsonian Tropical Research Institute, wondered whether some species would lean on other senses more heavily to accomplish the same task.

So he and his research colleagues captured 12 fringe-lipped bats and brought them to an outdoor flight cage at a lab in Panama. The humans trained the bats to attack robot frogs that emitted recorded tungara mating calls, which sound something like a slide whistle played on a 1980s synthesizer.

On their perches, the bats passively listen for this call, but that's not all they can tune in to.

While the frogs ribbit away on the rainforest floor, they also inflate and deflate their vocal sacs. That gives the fringe-lipped bats an opportunity to echolocate the motion of the frogs' throats.

"It's the same way we humans talk to each other," said Wouter Halfwerk, an evolutionary ecologist at Vrije Universiteit Amsterdam who worked with Gomes. "We pay attention to the sounds, but we also look at lip movement."

To see how noise affects the bats' ability to hunt, the researchers placed two robofrogs in the lab's flightcage. Some of the "prey" had inflating vocal sacs, and some did not.

The plastic robot frog with an inflating vocal sac.
The plastic robot frog with an inflating vocal sac. (Rachel Moon)

At times during the experiment, the researchers introduced computer-generated white noise that overlapped with the frequency of the frogs' calls. That made it more difficult for the bats to locate the robofrogs using sound alone.

The experiment revealed that, compared with bats that didn't have to deal with noise pollution, the distracted bats doubled their use of echolocation and clearly preferred to attack the robofrog with the moving throat. The bats with unimpeded hearing attacked either type of robofrog at random.

The findings, published Thursday in the journal Science, are the first to show an animal choosing between one mode of perception (hearing) and another (echolocation) as an adaptation to conditions in their environment.

This ability to shift strategies doesn't mean the bats are in the clear. In the presence of white noise, the fringe-lipped bats spent more time flying from perch to prey. In the wild, this would give frogs a better a chance to escape, the study authors wrote.

A bat attacks a robofrog inside an outdoor flightcage. Researchers used pieces of fish as a reward to train the wild animal to go after the stand-in amphibian. (Barrett Klein and Andy Quitmeyer)

"As sources of anthropogenic [human-generated] noise continue to expand, animals will, ultimately, have to face noise in one way or another," Gomes said.

Recent studies have shown that noise pollution reduces the survival and reproduction rates of some animal species — including making bats and owls less effective hunters. While the fringe-lipped bats can shift from one sense to another and continue foraging under noisy conditions, other species may not be able to do this, Gomes said.

Gomes said it is particularly important to understand how humans' actions may affect bats. Farmers rely on bats to pollinate bananas, peaches and other cash crops. They're also the only natural pollinator of agave, which is used to make tequila.

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At an ecosystem level, noise pollution could potentially reshuffle the animals found in certain areas based on those that can adapt and those that cannot.

"It is likely that many animal communities would be altered by anthropogenic noise," Gomes said. "Animals rely so heavily on sounds to communicate, find food [and] avoid predators … much more than humans do — we are quite visual creatures."

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