We now know how horses whine
Tierfotoagentur/E. Hofstede/Alamy
When a horse whines, it makes two sounds at the same time, which no other animal can: a low sound made in the same way as a cow’s lowing, and a high-pitched whistle in the throat.
It has long been known that a horse’s whinny contains both a low-frequency sound, around 200 hertz, and a high-frequency sound, over 1000 hertz – a phenomenon known as biphonation. The low-frequency component is easily explained by vibrations in the horse’s larynx-vocal fold, similar to human speech or singing. But the high-frequency sound is unusual for such a large animal, and how it is produced has been a mystery.
“Although humans have existed and evolved alongside horses for 4,000 years, we still imperfectly understand their communication,” says Tecumseh Fitch of the University of Vienna, Austria.
To solve the puzzle, Fitch and his colleagues first performed a series of tests, scans and experiments on horse throats obtained from a horsemeat supplier.
“We blew air through them, and at first we only got the low component,” says Fitch. “But with a little play, we were able to get the high-frequency component as well.”
This showed that both components are produced by the larynx itself, unlike human whistling, which is performed with the lips.
For further evidence of the whistling mechanism, the researchers blew air and helium, two gases of different densities, through the larynx. By comparing these gases, the researchers could test whether the high-frequency sound came from whistling or from tissue vibrations.
“The low-frequency component is produced by vocal fold vibrations, just like human singing or a cow mooing, and it didn’t change when we switched between gases,” says Fitch. “However, the high-frequency component shifts significantly to be higher in helium, just as predicted for flutes.”
Endoscopic tests of live horses showed that the muscles around the larynx contract at the onset of a whine. This causes a narrowing of the glottis, the part of the larynx where the vocal folds are, tilting of the vocal folds and increasing airway resistance. It forces air through the slit-like opening at high speed.
Mice and rats can also produce laryngeal whistles, but in their case it is at a frequency too high for humans to hear.
“Horses are the only mammal known to use the larynx to produce two frequencies simultaneously, one of which is a whistle, and in fact they are the only large mammal, besides humans, that produces whistles as part of the standard vocal repertoire,” says Fitch.
“It is the first strong experimental evidence of an aerodynamic laryngeal whistle production in any animal outside the rodent family,” says Ben Jancovich of the University of New South Wales in Sydney, Australia.
Fitch and his colleagues speculate that the whistle may make the whine more noticeable and may help carry the sound, but these hypotheses have yet to be tested.
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