Bats grunt like death metal singers to communicate with roostmates

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What do the frontman from Cannibal Corpse and a Daubenton’s bat have in common? They both use distinct structures in their larynxes, or voice boxes, to produce booming, demonic vocalisations, a study carried out at the University of Southern Denmark has found.

Echolocating bats are known to have an incredibly wide vocal range. They can make sounds spanning around seven octaves – impressive considering even Mariah Carey can only manage five.

To investigate how they achieve such a feat of vocal gymnastics, the Danish team removed the larynxes from five adult Daubenton’s bats, Myotis daubentonid, mounted them in a frame, applied an airflow designed to mimic the animals’ natural vocalisations and filmed the movements with a high speed 250,000 frames per second camera. They then used machine learning models to reconstruct the motion of the bats’ vocal membranes.

They found that the bats use a specific structure in their voice boxes known as false vocal cords that are not used in normal vocalisations to lower the frequency of their calls. The bats lower the false vocal folds down so that they oscillate together with their regular vocal cords. The additional weight this provides significantly lowers the pitch of the call. It is the same technique used by death metal singers to produce their trademark guttural growls.

The pitch of the growls is between 1 and 5kHz – the same pitch as the highest two octaves of a standard piano. Sounds bats use for echolocation can reach frequencies of up to 120kHz.

The bats often make the growling sounds when they fly in and out of a densely packed roost though it is unclear what their purpose is.

“Some seem aggressive, some may be an expression of annoyance, and some may have a very different function. We don’t know yet,” said the study’s co-author Lasse Jakobsen.

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