Bats growl like a metal singer to communicate with each other, study reveals

It's not only the singers who are prone to perform death metal. Believe it or not, but also bats are terribly willing to do, according to new research. Recently published in PLOS Biology today, the study led by Coen Elemans at the University of Southern Denmark and his team.

So, why can bats make this sound? Bats can use distinct structures in the larynx  — a hollow tube that connects your throat to the rest of your respiratory system — to produce high-frequency echolocation calls and lower-frequency social calls. Therefore, the low-pitched calls sound like death metal vocalists' growls.

"We identified for the first time what physical structures within the larynx oscillate to make their different vocalizations. For example, bats can make low-frequency calls, using their so-called “false vocal folds” – like human death metal singers do," said Coen Elemans in the statement.

They have the 7-octave vocal range

Unlike humans or some mammals, Daubenton’s bats have 7-octave vocal ability. This means that they can produce as much sound as a piano can. They are the only mammals whose echolocating and social cries have a frequency range between 1 and 120 kilohertz.

Researchers removed the larynxes from five adult Daubenton's bats, mounted them, and filmed them at 250,000 frames per second while applying a flow of air to simulate natural vocalization to better understand how different vocal structures enable bats to produce such a wide range of calls. The mobility of vocal membranes that were hidden by other components was then recreated using machine learning.

To produce high-frequency echolocation calls, they discovered that air pressure caused self-sustaining vibrations in the vocal membrane at frequencies between 10 and 70 kilohertz. However, the animals' lower-frequency social calls are likely produced by thick folds of a membrane known as the "ventricular folds," which were found to vibrate at frequencies between 1 and 3 kilohertz right above the vocal cords.

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

Incredibly high-frequency echolocation cries

The study also provides the first explanation of how bats produce their incredibly high-frequency echolocation cries. They achieve this by vibrating their extremely thin vocal membranes, which were initially present in humans but disappeared throughout our evolutionary process.

"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. 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, " said Jonas Håkansson, another author of the study.

Study abstract

Echolocating bats produce very diverse vocal signals for echolocation and social communication that span an impressive frequency range of 1 to 120 kHz or 7 octaves. This tremendous vocal range is unparalleled in mammalian sound production and thought to be produced by specialized laryngeal vocal membranes on top of vocal folds. However, their function in vocal production remains untested. By filming vocal membranes in excised bat larynges (Myotis daubentonii) in vitro with ultra-high-speed video (up to 250,000 fps) and using deep learning networks to extract their motion, we provide the first direct observations that vocal membranes exhibit flow-induced self-sustained vibrations to produce 10 to 95 kHz echolocation and social communication calls in bats. The vocal membranes achieve the highest fundamental frequencies (fo’s) of any mammal, but their vocal range is with 3 to 4 octaves comparable to most mammals. We evaluate the currently outstanding hypotheses for vocal membrane function and propose that most laryngeal adaptations in echolocating bats result from selection for producing high-frequency, rapid echolocation calls to catch fast-moving prey. Furthermore, we show that bats extend their lower vocal range by recruiting their ventricular folds—as in death metal growls—that vibrate at distinctly lower frequencies of 1 to 5 kHz for producing agonistic social calls. The different selection pressures for echolocation and social communication facilitated the evolution of separate laryngeal structures that together vastly expanded the vocal range in bats.