Scientists finally solve the mystery of the horse whinny

"We now finally know how the two fundamental frequencies that make up a whinny are produced by horses," says author Elodie Briefer of the University of Copenhagen. "In the past, we found that these two frequencies are important for horses, as they convey different messages about the horses' own emotions. We now have compelling evidence that they are also produced through distinct mechanisms."

Why Horse Vocal Communication Is So Unusual

Although horses have lived alongside humans for over 4,000 years, scientists still have much to learn about how they communicate through sound. In most large mammals, deeper voices are expected because the larynx typically grows larger as body size increases. Bigger animals usually produce lower pitched calls. Horses, however, do not completely follow this pattern, since their whinnies include unexpectedly high frequencies.

To understand how this happens, researchers examined the physical processes behind the whinny. They discovered that it involves a rare phenomenon known as "biphonation," meaning a single call contains two independent frequency components, one low and one high.

How Horses Produce High and Low Frequencies

The low frequency portion of the whinny comes from vocal fold vibration, much like a person singing or a cat meowing. Until now, the source of the high frequency component was unclear. To solve this, the team analyzed the horses' vocal anatomy, reviewed clinical data, and carried out detailed acoustic studies.

"Solving this biomechanical puzzle required combining approaches from veterinary medicine to acoustic physics," says author Romain Lefèvre of the University of Copenhagen.

Their findings showed that the high frequency element is produced by a laryngeal whistle. This mechanism works in a way that is similar to a human whistle, except the airflow that creates the sound becomes turbulent inside the horse's larynx rather than at the lips. Small rodents such as rats and mice are known to produce laryngeal whistles, but horses are the first large mammal identified using this method. They are also the only animals known to whistle in this way while simultaneously vibrating their vocal folds.

Helium Experiments Confirm the Laryngeal Whistle

To confirm the source of the high frequency sound, researchers conducted excised larynx experiments. They passed air through larynges removed from deceased horses and then alternated the airflow between normal air and helium. Because sound travels faster in helium, whistle frequencies shift higher when helium is used, while sounds created by vibrating vocal folds remain unchanged.

The results matched their predictions. When helium flowed through the larynx, the high frequency component rose in pitch, but the low frequency component stayed the same.

"When we blew helium through the larynges for the first time, the frequency shift was immediately obvious, and we knew we'd solved the mystery," says author William Tecumseh Fitch of the University of Vienna. "We were thrilled!"

Evolution of Biphonation in Horses

These findings clarify how the two overlapping pitches, or biphonation, are physically produced. The researchers suggest that this vocal ability likely evolved so horses can communicate multiple independent signals at once.

They also found that Przewalski's horses, a species closely related to domesticated horses, produce whinnies that include biphonation. In contrast, more distant relatives such as donkeys and zebras appear to lack the high frequency component. This difference suggests that horses developed specialized vocal adaptations, giving them a broader and more complex range of calls compared to other mammals.

"Understanding how and why biphonation has evolved is an important step towards elucidating the origins of the amazing vocal diversity of mammalian vocal behavior," says author David Reby of the University of Lyon/Saint-Etienne.

This work was supported by the Swiss National Science Foundation, the Austrian Science Fund, and Institut Universitaire de France.