How Horses Whinny: Whistling While Singing

"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."

While domesticated horses have lived closely with humans for over 4,000 years, horse vocal communication is poorly understood. Larger mammals generally make lower-pitched sounds because the size of the larynx typically increases in step with body size. But researchers have noted exceptions to this rule-and horses' whinnies are a prominent example.

To learn more, the team sought to explore the mechanics behind horses' whinnies. They found that the whinny represents an unusual vocal phenomenon, known as "biphonation," in which a vocalization has two independent frequency components: low and high.

While the low frequency is made by vocal-fold vibration, just like a human singing or a cat meowing, the origin of the high-frequency component has remained mysterious until now. To learn more about this high-frequency sound, the team gathered a wide range of data through careful study of the animals' vocal anatomy, clinical data, and acoustic analysis.

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

They found that the high-frequency component of the whinny is generated by a laryngeal whistle. The researchers describe it as similar in principle to a normal human whistle, except that the turbulent airstream that creates the whistle sound is created within the horse's larynx. Although some small rodents like rats and mice produce laryngeal whistles, horses are the first large mammal species found to whistle in this way and the only animals known to do so simultaneously with vocal-fold vibration.

To demonstrate this whistle, the researchers performed excised larynx experiments, blowing a stream of air through the larynx removed from deceased horses. While doing so, they switched the airstream into the larynx from air to helium and back again. They explained that because the speed of sound is higher in helium, it causes whistles to shift to higher frequencies, while the frequency of normal vocal-fold vibration remains unchanged. As predicted, the high-frequency component of the whinnies shifted upward when using helium, while the low frequencies did not change.

"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!"

The new findings help to explain how the two overlapping pitches-or biphonation-occur, says the team. They suggest that horses' biphonation likely evolved to convey multiple independent messages concurrently.

The researchers also note that Przewalski's horses-a species closely related to the domesticated horse-also produce whinnies with biphonation, but more distant horse relatives like donkeys and zebras appear to lack the high component of the whinny, suggesting that horses have unique vocal adaptations which allow them to produce a richer and more complex spectrum 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.