Every time Sofia Agnifilo walks into the field at her farm in the Berkshires, her 3-year-old Welsh pony, Spoc, greets her with a high-pitched, vibrating whinny -- usually once or twice -- clearly aimed at her, not the other horses.
“He only does it at people,” Agnifilo told ABC News. “He makes that sound when he’s happy to see us and wants attention.”
Like Agnifilo, most horse owners know their equine friends have an impressive vocal range. But the whinny -- a ringing, attention-seeking call -- is so unique that it has caught the attention of scientists.
A horse’s whinny showcases the remarkable adaptive flexibility of the equine vocal production system, Elodie Briefer of the University of Copenhagen, lead author of a new Current Biology paper on horse communication, told ABC News.
“And now we finally know how the two fundamental frequencies that make up a whinny are produced by horses,” she said.
The whinny is a combination of two distinct sounds at the same time, a rare talent for mammals known as biphonation, she explained.
One tone is a low baritone, around 200 hertz, which fits with what scientists expect from a large animal. The other is surprisingly high -- often above 1,000 hertz -- about the same pitch as a tea kettle whistle.
For a roughly 1,100-pound animal, that top note is a surprise, the researchers noted. Based on well-established models, a horse’s vocal folds can’t stretch fast enough to get into the soprano range.
Instead, researchers found that the upper pitches appear to form when air rushes through a narrow opening in the animal’s larynx, creating a tiny jet of turbulent air.
To find out exactly how horses pull off these impressive acoustical gymnastics, Elodie and her team ran a series of unusual experiments. First, they studied the throats of equine cadavers in the lab by pushing air, then helium through them.
When researchers switched to helium, the sound frequency shifted upward while the low tone did not. That told them the high sound behaves more like a whistle rather than a song-like vibration.
Scans of horse throats offered more clues.
Investigators next examined horses with a condition that partially paralyzes one vocal fold and they found that the low tone often broke down or disappeared. The high tone remained intact, which the researchers took as solid evidence that the two sounds are emitted from separate systems.
This song-and-whistle combo appears to be a talent reserved for horses and their closest wild relatives, the researchers said. Donkeys and zebras, despite belonging to the same equine family, can’t reach into the same Mariah Carey–worthy range.
“In the past, we found that these two frequencies are important for horses, as they convey different messages about the horses’ own emotions,” Elodie said.
One tone seems to reflect how positive or negative the emotion feels, while the other signals how intense or urgent it is, she explained.
That ability may help other horses, as well as their human friends, quickly assess the caller’s mood and level of arousal, which could shape how they respond.
But Agnifilo said she didn’t need a lab study to interpret what her horses are saying, even when they’re using two voices at the same time.
“They’re having fun,” she said.