When blasted with a cacophony of manmade noise, these hunting bats have no problems adapting. They'll switch from eavesdropping to echolocation, deftly choosing whichever hunting strategy works best.

A team of biologists has just discovered that a certain species of bats are masters at stalking prey through artificially noisy environments, like areas around cities and near highways. Led by Wouter Halfwerk, an animal ecologist at VU University in the Netherlands, the scientists made their discovery by crafting a fascinating experiment using 12 bats, a large cage strung with microphones and speakers, and a handful of inflatable robotic frogs. They break down the results of their experiment today in the journal Science.

"On average, we found that bats will send out roughly twice as many ultrasonic [echolocation calls] when in these noisy environments," Halfwerk says. "Because we can listen to and track these calls, that gives us a detailed picture of which senses the bats are relying on as they hunt."

Inflatable Robot Frogs

Wiki Commons / Karin Schneeberger

Halfwerk and his colleagues were studying a species of bat called fringe-lipped bats, native to Central and South America. Like North America's pallid bat or Europe's long-eared bats, these fringe-lipped bats hunt by eavesdropping on their prey rather than relying solely on echolocation—the process through which bats perceive objects via the echoes of their own ultrasonic chirps.

Fringe-lipped bats hunt certain frogs that send loud, croaking mating calls at night. Halfwerk's team wanted to know how these bats would respond if those mating calls were obscured by background noise. So, he and his colleagues set up an experiment where cage-bound bats would be subjected to a din of artificial noise while they tried to track and hunt a pair of stationary, robotic frogs.

Both robotic frogs produced a loud recorded croak, but the scientists made it so only one of the frogs could be easily detected with echolocation. That's because just one of the robot frogs would be inflating and deflating a fake, silicone balloon in its neck. In the wild, frogs naturally inflate a vocal sac as they croak and bats can pick up this movement via echolocation.

"Because we can listen to and track these calls, that gives us a detailed picture of which senses the bats are relying on as they hunt."

Like the real thing, these inflatable robotic frogs were pretty small—less than an inch long. To inflate their silicone necks, Halfwerk's team ran a small tube of pressurized air to the robots from outside the cage. To make sure the bats didn't tire of hunting fake, inedible prey, every third run of the experiment a small piece of fish was placed on top of the frogs.

Listen and Strike

Halfwerk's team let their bats hunt the robotic frogs under a few different noisy conditions in the large cage, all the while tracking and recording their ultrasonic chirps. When the cage was filled with ambient, natural noise, bats would hunt and attack both the stationary and inflating robots frogs indiscriminately. It didn't matter, they both made noise. But once the scientists turned on a cacophony of artificial noise, the bats would send out twice as many echolocative calls and would heavily prefer hunting the inflating frog rather than its still twin. That showed that the bats were using those extra calls to find prey.

Rachel Moon

According to Halfwerk, this shows that the bats are "pretty flexible with their hunting strategy," he says, "and it might explain why some species are much better at adapting to the noisy conditions around areas like highways or cities, which are heavily polluted with [man-made] noise."

This switch between eavesdropping and echolocation is pretty intuitive. We should know, Halfwerk says. "We humans experience things like this all the time. Most people, when trying to listen to someone in a noisy bar or train, will adapt and adopt different perceptual strategy, like paying more attention to lip movements."

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