Bat out of hell: Bats ‘growl like death metal singers’

In order to produce some of the tones in their extensive vocal range, bats use techniques similar to those used by human death metal singers, new research reveals.

Scientists at the University of Southern Denmark were able to examine, in detail for the first time, exactly what happens in a bat’s larynx when it produces sound.

The bats reportedly resort to "growling", producing low frequency sounds in the style of death metal singers, when they fly in and out of densely-packed roosts, with researchers suggesting it could be deployed to show aggression.

"We identified for the first time what physical structures within the larynx oscillate to make their different vocalisations. For example, bats can make low frequency calls, using their so-called ‘false vocal folds’ – like human death metal singers do," said Professor Coen Elemans from the university’s Department of Biology.

False vocal folds are called so because they look like vocal folds but they are not used in normal human speech and song. Only death metal growlers and throat singers from a few cultures around the world use their false vocal folds like the bats. Humans move the vocal folds down so that they oscillate together with the vocal folds.

"This makes the vocal folds heavy and therefore they vibrate at very low frequencies", said Jonas Håkansson, first author of the study.

Speaking about the bats’ low vocal range, biologist and bat expert Lasse Jakobsen, also from the University of Southern Denmark, and a co-author of the paper, said: "Some seem aggressive, some may be an expression of annoyance, and some may have a very different function. We don’t know yet".

In order to study how the bats’ bodies were able to produce the sounds, five larynxes from Daubenton’s bats were extracted, mounted and filmed while applying air flow to mimic natural breathing.

The high-speed videos the team took reveal vocal membranes and false vocal folds vibrating at different frequencies.

Their wide range of vocalisations also form a vital part of bats’ hunting and navigating abilities.

When bats hunt insects in complete darkness, they use echolocation.

They emit very short, high frequency calls, and listen for echoes reflected from objects in the surroundings to detect and capture insects.

"A bat can determine the shape, size and texture of echoing objects within milliseconds," said Dr Jakobsen.

The study also reveals for the first time how bats are able to make their extraordinarily high frequency echolocation calls.

They do this by vibrating very thin vocal membranes – structures that humans also once had, but were lost in our evolution.

"We have directly filmed these vocal membranes for the first time. To show their vibrations we needed to film at extremely high rates, up to 250,000 frames per second," said Dr Håkansson.

"We see many adaptations in the larynx that we think are responsible for the bat’s ability to make very high frequency calls very fast, so that they can catch insects while flying."

Together the normal vocal range for a bat spans seven octaves, the research team said.

"That is remarkable. Most mammals have a range of three to four, and humans about three," said Professor Elemans.

He added: "Some human singers can reach a range of four to five, but they are only very few. Well-known examples are Mariah Carey, Axl Rose and Prince.

It turns out that bats surpass this range by using different structures in their larynx."

The research is published in the journal Plos Biology.