A horse's neigh may be unique in the animal kingdom. Now scientists know how they do it

NEW YORK (AP) — Horses whinny to find new friends, greet old ones and celebrate happy moments like feeding time.

How exactly horses produce that distinctive sound — also called a neigh — has long eluded scientists.

The whinny is an unusual combination of both high and low pitched sounds, like a cross between a grunt and a squeal — that come out at the same time.

The low-pitched part wasn't much of a mystery. It comes from air passing over bands of tissue in the voice box that make noise when they vibrate. It's a technique similar to how humans speak and sing.

But the high-pitched piece is more puzzling. With some exceptions, larger animals have larger vocal systems and typically make lower sounds. So how do horses do it?

According to a new study, they whistle.

Researchers slid a small camera through horses' noses to film what happened inside while they whinnied and made another common horse sound, the softer, subtler nicker. They also conducted detailed scans and blew air through the isolated voice boxes of dead horses.

The whinny's mysterious high-pitched tones, they discovered, are a kind of whistling that starts in the horse's voice box. Air vibrates the tissues in the voice box while an area just above contracts, leaving a small opening for the whistle to escape.

That's different from human whistling, which we do with our mouths.

“I'd never imagined that there was a whistling component. It's really interesting, and I can hear that now,” said Jenifer Nadeau, who studies horses at the University of Connecticut. Nadeau was not involved with the study, which was published Monday in the journal Current Biology.

A few small rodents like rats and mice whistle like this, but horses are the first known large mammal to have a knack for it. They're also the only animals known to be able to whistle through their voice boxes while they sing.

“Knowing that a ‘whinny’ is not just a ‘whinny’ but that it is actually composed of two different fundamental frequencies that are created by two different mechanisms is exciting,” said Alisa Herbst with Rutgers University's Equine Science Center, of the study in an email.

A big lingering question is how horses' two-toned calls came to be. Wild Przewalski’s horses can do something similar, as can elks. But more distant horse relatives like donkeys and zebras can't make the high-pitched sounds.

The two-toned whinnies could help horses convey multiple messages at the same time. The differently pitched neighs may help them express a more complex range of feelings when socializing, said study author Elodie Mandel-Briefer with the University of Copenhagen.

“They can express emotions in these two dimensions,” Mandel-Briefer said.

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Associated Press video journalist James Brooks contributed to this report.

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