Bats are known for making high-pitched calls that they use for echolocation. But bats are also capable of producing extremely low-pitched growling sounds much like the snarling vocals of death metal singers — and now, scientists know how bats do it.
Like death metal vocalists, bats achieve these low frequencies by using what are known as false vocal folds, said Jonas Håkansson, a postdoctoral researcher who studies bat vocalization at the University of Southern Denmark in Odense and the University of Colorado in Colorado Springs.
"What helps them growl are the ventricular folds, also called false vocal folds, that sit above the true vocal folds," he told Live Science. False vocal folds are thick folds of mucous membrane that appear in the larynxes of most mammals; "these vibrate at a comparatively low frequency and thus produce audible sounds (growls)," Håkansson explained. Such sounds are hardly ever uttered by humans — except by trained Mongolian throat singers and, of course, death metal vocalists.
And now, bats can be added to the list of guttural performers.
Researchers recently examined this unusual vocal ability in Daubenton's bats (Myotis daubentonii), which live across Europe and Asia and have a wingspan of about 9.8 inches (25 centimeters), according to Animal Diversity Web. The scientists reported their findings Nov. 29 in the journal PLOS Biology.
To understand the vocal range of these tiny bats, the researchers captured the first-ever footage of bat vocal cords in action, using extracted bat larynxes that they animated with flowing air to simulate bronchial pressure. They then filmed the larynxes at rates of up to 250,000 frames per second. The high-speed footage revealed that sounds produced by the false vocal folds were very low-pitched, in the range of 1 to 5 kHz.
Related: Bats tell predators to 'buzz off' — literally
The research team also learned that the bats' vocal range was broader than they expected, spanning seven octaves. For comparison, humans — and most other mammals — can manage only three or four octaves. (Singers such as Prince, Mariah Carey and legendary Queen frontman Freddie Mercury, who have ranges of four to five octaves, are rare exceptions.) What gives the bats a high-frequency boost is membranes that extend from the vocal cords and measure no more than 0.0004 inch (10 micrometers) thick — a feature that humans lack.
Some primates do have these larynx membranes, but humans are thought to have never developed them or lost them at some point in our evolutionary past, according to the study.
"The high-frequency calls used for echolocation are produced by the vocal membranes," Håkansson said. "These are thin membranes that sit at the end of the vocal folds. Due to their low mass, they can oscillate at very high frequencies and thus produce the high-frequency calls," which the scientists measured at frequencies ranging from 10 to 20 kHz. The combination of these delicate membranes and thicker folds is what allows bats to display such an impressive range in their vocalizations, he said.
Why bats make death metal growling noises is not yet understood, the scientists reported. However, Håkansson and his colleagues noted that the bats would start to growl when they were crowded together, perhaps expressing aggression or annoyance.
"If you handle bats, like when netting, or when you observe them in groups, they will make these sorts of sounds," said Håkansson, though the exact reason remains a mystery.
Nevertheless, it's still pretty metal.
