Researchers have discovered that horses produce whinnies using two distinct mechanisms simultaneously: a low tone from vibrating vocal folds and a high-pitched whistle from the larynx. This biphonation allows horses to convey multiple emotional signals in one call. The findings, published on February 23 in Current Biology, mark the first confirmation of this process in a large mammal.
Horses have coexisted with humans for more than 4,000 years, yet the mechanics of their vocalizations remain partly mysterious. A new study reveals that a horse's whinny combines a deep frequency, generated by vibrating vocal folds similar to human singing, with a high-pitched sound produced by a laryngeal whistle. This phenomenon, called biphonation, involves turbulent airflow inside the larynx, akin to a human whistle but occurring internally.
The research team, including Elodie Briefer from the University of Copenhagen, analyzed vocal anatomy, clinical data, and acoustics to identify these processes. "We now finally know how the two fundamental frequencies that make up a whinny are produced by horses," Briefer stated. "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."
To verify the high frequency's origin, scientists conducted experiments on excised larynges from deceased horses. They passed air through the larynges, then switched to helium, which travels faster and shifts whistle frequencies higher while leaving vocal fold vibrations unchanged. The results confirmed the predictions, with the high pitch rising in helium but the low tone remaining steady.
"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," said William Tecumseh Fitch from the University of Vienna. Romain Lefèvre from the University of Copenhagen noted that solving this required integrating veterinary medicine and acoustic physics.
Biphonation appears in Przewalski's horses, close relatives of domesticated horses, but not in donkeys or zebras. This suggests specialized vocal adaptations in horses, enabling more complex communication. "Understanding how and why biphonation has evolved is an important step towards elucidating the origins of the amazing vocal diversity of mammalian vocal behavior," commented David Reby from the University of Lyon/Saint-Etienne.
The study was supported by the Swiss National Science Foundation, the Austrian Science Fund, and Institut Universitaire de France. It appears in Current Biology (2026; 36(4):902), with DOI: 10.1016/j.cub.2026.01.004.