If you've ever listened to death metal rock or Mongolian throat singing, you'll be familiar with the growling vocals that are part of both. Well, bats make similar sounds, and it turns out that they do so in the same manner as human singers.
When vocalists in bands such as Morbid Angel or The Hu sing/growl the lyrics to their songs, they utilize membranes in their throat known as false vocal folds (aka ventricular folds).
These are located above the "true" vocal folds (aka vocal cords), and they look much like them. Unlike the true folds, however, the false folds aren't utilized in regular human speech or singing. That said, by moving their false vocal folds downward, singers are able to make them oscillate in unison with the true vocal folds – the resulting very-low-frequency false vocal fold vibrations produce the growling sound.
For some time now, it has been known that bats make low-frequency growls of their own, although the means by which they do so was unclear.
In order to find out, scientists from the University of Southern Denmark recently extracted the larynxes from five dead adult Daubenton’s bats (Myotis daubentonii), then filmed them at 250,000 frames per second as air was blown through them. The air flow caused structures in the larynxes to vibrate, much as they would in natural bat vocalizations.
Upon analyzing the slow-motion footage, it was found that while thin vocal membranes in the larynxes vibrated at high frequencies to produce the chirps used in echolocation, the false vocal folds vibrated at much lower frequencies, producing the growling sounds. The purpose of the growls still isn't entirely understood, although it's likely social – bats tend to make the noises as they fly in and out of densely packed colonies.
All told, from their highest chirps to their lowest growls, bats have a vocal range of seven octaves – which is a lot.
"That is remarkable," said the lead scientist, Prof. Coen Elemans. "Most mammals have a range of three to four, and humans about three. 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."
A paper on the research was recently published in the journal PLoS Biology. You can hear some of the bat growls in the video below – they may sound surprisingly chirpy, but Elemans assures us that they're quite low for bat vocalizations.
Sources: University of Southern Denmark, PLoS
