Bats growl like heavy metal frontmen

How the special biology of the bat's larynx helps them bellow low growls and high pitched squeals.....
09 December 2022

Interview with 

Coen Elemans, University of South Denmark

BATS

A colony of bats in the air.

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Listeners of a certain vintage may recall the shocking moment on January 20th, 1982 when heavy metal singer Ozzy Osbourne bit the head off a bat live on stage. But a new study has shown that bats and singers might have more in common than just that unfortunate incident. For the first time, scientists have been able to study the workings of the bat voicebox, or larynx, to watch how the vocal cords, as well as sets of additional vocal cords - called “false vocal folds” - vibrate to produce sounds, both in bats, and in us: it’s how Tuvan throat singers make those haunting melodies. Will Tingle spoke with the University of Southern Denmark’s Coen Elemans…

Coen - I'm generally interested in how animals produce sounds and so we've studied this over the last decade or so in lots of different animals and we typically see that most animals use something similar as we do. So they have vocal folds in their larynx or birds in the syrinx and they oscillate these tissues basically and that produces sound. And in bats there was a long standing debate how they actually make these sound. The idea was that they make it by very thin membranes that sit on their vocal folds, but we weren't quite sure because it's very hard to film this. So if you want to have direct evidence, you need to film this in a tiny bats tiny larynx. And the oscillations they make go over a hundred thousand times a second. So you need very fast filming and you need then lots and lots of light. So this is very difficult. You can't ask a bat like 'open your mouth, echolocate, and we'll stick a camera in that's <laugh> very large and that needs lots of light'. So for a long time people haven't been able to solve this. And what we did now is we used a very different approach. So we studied basically the isolated larynx and then we can very nicely film everything. And that was the incentive of the study to figure out how bats make sound.

Will - And what did your study do to try and achieve this?

Coen - What we did is we studied the isolated larynx and that allows us to go to very high frame rates. So we filmed these vocal fold oscillations at almost a quarter million frames per second. So that's a lot. And then we could very nicely see how these vocal membranes that sit on the top of the vocal folds oscillate and produce the echolocation.

Coen - When we did that, we also saw that slightly above the vocal folds is another set of vocal folds that we also have that's called the false focal folds. And they're called false vocal folds because in humans they've never found really any function when they're originally described them. They're not used in normal speech, they're not used in normal song, but we found in bats that they're oscillating very nicely, very easily. And they did so at a frequency that corresponds to their social calls. So when they are for example, annoyed with each other or when they return from a flight into a colony, they use these calls to welcome each other.

Coen - But we saw that the false vocal folds do this. And in humans, the only recorded evidence for common use of these false vocal folds is in death metal grunting where we have some evidence, not a lot either. And then for example, Tuvan throat singers also seem to to lower their false vocal folds close to their normal focal folds and then they oscillate together and that gives them a very low frequency, but it also gives them a very rough sound because they typically, the oscillations become very irregular.

Will - Presumably then we didn't evolve these false vocal chords for purely the process of death metal singing. So it's perhaps something to be considered as a vestigial limb almost, maybe had a common ancestor.

Coen - Yeah, so very little is known about these false vocal folds and their function in sound production. So yeah, we don't know actually that's a good idea. I like it.

Will - And what did your study find?

Coen - So what we found is that the vocal membranes oscillate these very high frequencies and generates the echolocation calls. The echolocation is what makes bats so special. They evolved flight and they evolved echolocation so they can hunt at night very fast moving prey. And then we found that the false vocal folds oscillate at these lower frequencies and together they give the bats an enormous vocal range So they can produce very low growls and that are around one kilohertz. That's very low for them because it's a small animal and they can go all the way up to 120 kilohertz with their vocal membranes. And that gives you a vocal range of about seven octaves, which is absolutely enormous considering that normal vertebrates and normal humans have an octave range of about two or three, really good singers with a large vocal range four. And like the absolute top singers like Mariah Carey and Prince and David Bowie, they can go up to almost five. But that's exceptional and this is a normal bat.

Will - Well, so we should be looking out for our next hot, new single will be from a bat then <laugh> Christmas number one.

Coen - Yeah, exactly.

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