How Do Horses Whinny? .

 

An equine makes the low-pitched part of its whinny by vibrating its vocal cords—similar to how humans speak and sing—and the high-pitched part by whistling - 

 

Sarah Kuta - Daily Correspondent - Smithsonian 

A horse’s whinny is a distinctive and instantly recognizable sound, which the animals use to communicate with each other and express emotions. But this unique vocalization has long puzzled scientists, since it includes both high- and low-pitched components.

Now, researchers say they may have solved this equine mystery. A horse makes the low-pitched part of its whinny by vibrating its vocal cords—similar to how humans speak and sing—and the high-pitched part by whistling with its voice box, scientists report in a new paper published February 23 in the journal Current Biology. This marks the first known example of a creature that can make these noises simultaneously.

The whistle came as a bit of a surprise. To make this sound, a horse contracts the muscles around its voice box, or larynx, and forces air through a small, slit-like opening. Mice and rats can do this too, though their vocalizations are too high-pitched for humans to hear.

“It’s the first strong, experimental evidence of an aerodynamic laryngeal whistle production in any animal outside the rodent family,” says Ben Jancovich, a biologist at the University of New South Wales in Australia who was not involved with the research, to New Scientist’s James Woodford.

Humans tamed the ancestors of most modern domestic horses around 4,200 years ago, and we’ve been living closely with the creatures ever since. But researchers still don’t fully understand how the animals communicate.

In 2015, scientists reported that a horse’s whinny contains two distinct sounds—one high-pitched, one low-pitched—that the animal produces simultaneously. The realization that horses can make high-frequency sounds was perplexing, because vocal pitch tends to be related to body size in a specific way: Large animals generally make lower-pitched sounds, whereas small animals make higher-pitched ones.

“The rule of thumb is that the lower the frequency the animal can produce, the more the animal sounds like, ‘I’m fit, I’m big, I’m bad, you don’t want to mess with me,’” Peter Scheifele, a neuroaudiologist at the University of Cincinnati who was not involved with the new research, tells Amy Briggs of the Australian Broadcasting Corporation.

For the new study, scientists decided to probe this trend-bucking phenomenon even further. They set out to understand the physical mechanisms at play in a horse’s two-toned whinny.

Scientists slid an endoscopic camera through live horses’ noses to get an up-close look at their larynxes as they made sounds. The animals’ vocal cords vibrated whenever they made lower-pitched sounds, but didn’t move when they produced higher-pitched ones, reports the New York Times’ Kate Golembiewski. But during high-pitched vocalizations, the muscles above the vocal cords strongly contracted, per ABC.

Based on these results, the researchers suspected the high-pitched part of the whinny was an aerodynamic laryngeal whistle. To confirm that hunch, they blew compressed air and helium through larynxes removed from horses that had died, then compared the frequencies of the sounds produced by the two gases.

Noises made via whistle—in which a gas passes through or by a hole, creating tiny, high-speed swirls—should be different in helium compared to those in air, because the sounds travel at different speeds due to the gases’ distinct densities. Sounds produced by physical vibrations, by contrast, move at the same speed through both mediums.

As expected, the low-frequency part of the whinny was the same whether the researchers used air or helium. But the high-frequency component got much higher when they used helium, confirming the sound was indeed a whistle.

“I’d never imagined that there was a whistling component,” says Jenifer Nadeau, an animal scientist at the University of Connecticut who was not involved with the new study, to Adithi Ramakrishnan of the Associated Press. “It’s really interesting, and I can hear that now.”

The findings open the door for a wide variety of future research projects. For instance, are horses somehow controlling the whistle sound by altering the shape of their vocal tracts or by moving their tongues, jaws or lips? How and why did the creatures evolve this unique vocalization? Do they use the distinct elements of the whinny for different communication purposes?

“It could be that the high pitch is for long-distance communication and the low pitch for short distance,” study co-author Elodie Mandel-Briefer, a biologist at the University of Copenhagen in Denmark, tells ABC.