Bats growl like death metal singers to communicate with each other

Bats have an impressive vocal range, far surpassing that of humans, able to swing through seven octaves using their specially adapted voice box.

For the first time, researchers have filmed a bats' voice box, or larynx, as it produces sound, and found that they exploit a thick structure called "false vocal folds" to hit the lower end of their range. 

Humans have similar vocal folds that are not used in speech, but are employed by death metal singers and throat singers to make their guttural growls.

"I was interested in understanding how they actually produce the echolocation calls they have. And in doing so I realized they actually have an enormous range," said biologist Coen Elemans, who led the study. "In the ultrasonic range, up to 20 to 100 kilohertz, bats can produce sounds, but they can also make growls that are about one kilohertz."

"To us they still sound like squeaks, but for a very small animal like a bat it's actually a very deep sound."

The research was published in the journal PLOS Biology.

Filming vocal folds at 250,000 frames per second

Elemans and his team at the University of Southern Denmark initially focused on the bats' high frequency echolocation calls, which they emit to navigate the world around them using sound instead of sight. 

"Echolocation is still very fascinating," said Elemans. "[Bats] perform better than any man-made echolocation system or existing sonar system, and so there's a lot of incentive in trying to understand how the animals do this."

They extracted the larynxes from five deceased Daubenton's bats and mounted them in the lab. Then they blew air through the structures while filming at 250,000 frames per second, moving it around the way a bat would as it made vocalizations.

They discovered that echolocation calls were made by vibrating very thin membranes that sit on top of their normal vocal folds. But then they noticed another set of folds on their larynx were vibrating, which made low frequency sounds. These particular folds are found on the voice boxes of all vertebrates, but little is known about how they work.

"We have them too, and they're called false vocal folds … because in humans they have no function in normal speech," said Elemans. "We went back and thought a little bit about this like, who else uses these ventricular folds, these false vocal folds? And the only thing we could actually find is some evidence that death metal singers use them when grunting."

Grunts of annoyance

The researchers aren't sure what the bats are saying with these growls, but say they are very commonly heard around bat roosts by humans. "People have interpreted them as agonistic calls, or basically calls of aggression or annoyance, but we don't really know," said Elemans.

The researchers now believe that the bats adapted their false vocal folds because lower frequencies are better for communicating over long distances, whereas the high frequencies needed for echolocating are more short range.

"They are so heavily selected for echolocation that it has driven basically their sound production up into the high frequencies," he said. 

"What we think now is that bats adapted these false vocal folds to make these lower frequencies that are better for communication that radiates all directions and much further over several meters, while these echolocation calls die out within a meter and are very directional."

Next, Elemans wants to look at other animals that use echolocation such as toothed whales to see if they have a similar adaptation to their vocal folds.

Produced and written by Amanda Buckiewicz