Bats have been shown to use unusual and distinctive growls, akin to the sounds produced by death metal singers, to make social calls
Bats use distinct structures in the larynx to produce high-frequency echolocation calls and lower-frequency social calls, according to a study by Coen Elemans and colleagues at the University of Southern Denmark. The structures used to make the low-pitched calls are analogous to those used by death metal vocalists in their growls.
Echolocating bats have an extremely large vocal range of seven octaves, compared to just three to four octaves for most mammals, including humans. Their echolocating calls and social calls range between 1 and 120 kilohertz, making them unique among mammals.
To understand how different vocal structures allow bats to create such a wide range of calls, researchers extracted the larynx from five adult Daubenton’s bats (Myotis daubentonii), mounted and filmed them at 250,000 frames per second while applying a flow of air to mimic natural vocalisation. They then used machine learning to reconstruct the motion of vocal membranes that were obscured by other structures.
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aggressive bat calls
They found that air pressure generated self-sustaining vibrations in the vocal membrane at frequencies between 10 and 70 kilohertz, sufficient to produce high-frequency echolocation calls. In contrast, thick folds of membrane just above the vocal cords, called ‘ventricular folds’, vibrated at frequencies between 1 and 3 kilohertz, and are likely to be involved in producing the animals’ lower-frequency social calls.
Some humans also use their ventricular folds to produce low-frequency vocalisations, such as death metal growls and Mongolian throat singing.
The study is the first to directly observe self-sustained vibrations in bat vocal structures that can generate echolocation and social calls. Natural selection to produce high-frequency calls for echolocating prey and much lower-frequency calls for social communication have created distinct evolutionary pressures that expanded the vocal range of bats, the authors said.
They added: “We show that bats vibrate extremely thin and light membranes extending from their vocal folds to make their high-frequency ultrasonic calls for echolocation.
“To extend their limited lower vocal range, bats make aggressive calls with their ventricular folds – as in death metal growls.”
The research is published in PLOS Biology.
Daubenton’s bat (Myotis daubentonii) echolocating in flight. © Jens Rydell (CC-BY 4.0, https://creativecommons.org/licenses/by/4.0/).