Researchers at Johns Hopkins University have determined that bats waggle their heads and ears in synch with their sonar vocalizations. (Johns Hopkins University)

It's hard to describe just how cute a bat's "head waggle" is. To Melville Wohlgemuth, who has spent countless working hours watching the behavior, it looks kind of like his hard-of-hearing pet pug trying to focus on a noise. The bat cocks its head to the left, perks its large conical ears forward and squeaks, then tilts back toward the right.

You know what — words don't really do it justice. You should probably just watch:

(Mellville Wohlgemuth)
(Mellville Wohlgemuth)

"It's really adorable," Wohlgemuth said. "A lot of people have a negative impression of bats, but they're great little animals."

But Wohlgemuth, a behavioral neuroscientist at Johns Hopkins University, had a suspicion that the waggle was something more. He wanted to know what purpose it served. So he hooked his bats up to a motion capture system and microphone, set up a high-speed camera and tried to figure out what exactly was going on.

The results, published Thursday in the journal PLOS Biology, suggest that bats (and, to a lesser extent, dogs and humans) waggle their heads to enhance their perception of sounds as they survey their environments. The bats use the technique to help hunt for prey — in the words of one of his colleagues at Johns Hopkins, they "kill them with cuteness."

For the research, Wohlgemuth developed a test in which bats were seated on a platform while their prey — a small insect — flew toward them on a makeshift bug zipline. As the insects approached, the bats squeaked out a few vocalizations, which they use to echolocate. The vocalizations bounce off of surrounding objects and echo back at the bats, allowing them to "hear" their environment.

The whole time, "we could see this coordination of outgoing set of vocalizations with ear and head movements when echo returns," Wohlgemuth said. 

The bats were clearly moving their heads around to capture the sound bouncing off the incoming insect prey. But why?

Bat head-waggle sequence. (Johns Hopkins University)

When animals listen for something, they unconsciously pay close attention to the way the sound hits their left and right ears. The difference in the way each ear perceives the sound can help them sort out where it is coming from. For example, the buzz of a mosquito trying to bite your left shoulder will sound louder in your left ear than in your right, and that will help you figure out where to swat.

But by moving your head as you listen, you're slightly altering the orientation of each of your ears.

"You almost get a different snapshot of the sound coming in," Wohlgemuth said. "Sort of like how you might move your eyes around to get better views of the world."

All creatures with two ears do this — humans, cats, dogs. That's why Wohlgemuth recognized the movement from his dog. But the job of localizing sound is especially hard for bats: Their use of sonar means that they have sound coming in from all directions, all the time, so their waggle must be correspondingly thorough.

"They have to be able to find that tiny little echo of the insect against all the other clutter echoes of the background," Wohlgemuth said. "So bats have these very robust and very dramatic behaviors."

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