Bats are known for making high-pitched communication that they use for echolocation. But bats are also capable of producing very low growling sounds.
Bats achieve these low frequencies 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.
“What helps it growl is the ventricular folds, also called the false vocal folds, which lie above the true vocal folds,” he said. The false vocal folds are thick folds of mucous membrane that appear in the larynx of most mammals; “These vibrations vibrate at a relatively low frequency and therefore produce an audible growl,” Hakansson explained.
Researchers recently examined this unusual vocal ability in Dubenton 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 November 29 in the journal PLOS Biology.
To understand the vocal range of these tiny bats, the researchers captured the first footage of the bat’s vocal cords in action, using extracted bat larynxes that they swayed with flowing air to simulate bronchial pressure. Then they imaged the larynx at rates of up to 250,000 frames per second. The high-speed footage revealed that the sounds made by the false vocal folds were very low, in the range of 1 to 5 kHz.
The research team also learned that the bats’ vocal range was wider than they expected, extending to seven octaves. For comparison, humans — and most other mammals — can only manage three or four octaves. (Singers like Prince, Mariah Carey and Freddie Mercury, who have ranges of four to five octaves, are rare exceptions.) What gives bats a high-frequency boost is the membranes that extend from the vocal cords and measure no further than that. And it’s just over 0.0004 inch (10 micrometers) thick – a feature humans lack.
According to the study, some primates have these laryngeal membranes, but it is believed that humans never developed them or lost them at some point in our evolutionary past.
“The high-frequency communication used for echolocation is produced by the vocal membranes. These are thin membranes located at the end of the vocal folds. Due to their low mass, they can oscillate at very high frequencies, thus producing high-frequency communication,” which the scientists measured at frequencies of 10. to 20 kHz. He said the combination of these thin membranes and thick folds is what allows bats to display such a remarkable range in their vocalizations.
Häkansson and his colleagues noticed that the bats would begin to growl when they were crowded together, perhaps to express aggression or annoyance.
“If you handle bats, like when you’re setting up nets, or when you’re observing them in groups, they will make these kinds of sounds,” Hakansson said, though the exact reason remains a mystery.
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