Horses are unique in their vocalizations, producing two distinct sounds simultaneously—a low, cow-like moo and a high-pitched whistle from their throats.

Recent research reveals that horse vocalizations exhibit both low-frequency sounds around 200 hertz and high-frequency sounds exceeding 1,000 hertz, a phenomenon known as biphonation. The low-frequency sounds can be attributed to the vibrations of the horse’s laryngeal vocal folds, much like human speech and singing. However, the mechanism for producing the high-frequency sounds in such a large animal remains an intriguing question.

“Humans have coexisted with horses for over 4,000 years, yet equine communication remains partially misunderstood,” explains Tecumseh Fitch from the University of Vienna, Austria.

To investigate this phenomenon, Fitch and his team conducted tests and experiments on horse larynxes sourced from meat suppliers.

“Initially, we observed low-frequency sounds when we blew air into the larynx,” says Fitch. “However, after some adjustments, we successfully elicited high-frequency components.”

This established that both sound components originate from the larynx itself, contrasting with human whistling produced via the lips.

To confirm the whistling mechanism, researchers introduced air and helium—two gases of varying densities—through the larynx. This allowed them to test whether high-frequency sounds resulted from whistle production or tissue vibrations.

“The low-frequency sounds stem from vocal cord vibrations, similar to human singing or cow mooing, and remained unchanged when we altered the gases,” Fitch noted. “Conversely, the high-frequency content varied significantly with higher helium concentrations.”

Endoscopic examinations of live horses revealed that laryngeal muscles contract as the call begins, narrowing the glottis—where the vocal cords reside—and increasing airway resistance. This mechanism forces air through a narrow opening at high speed.

While other small mammals like mice and rats can produce laryngeal whistles, their frequencies are inaudible to humans.

“Horses are unique among mammals for their ability to produce two frequencies at once using their larynx, with whistles being part of their regular vocal repertoire,” Fitch adds.

This research sheds light on a previously undocumented method of aerodynamic whistle production in animals beyond rodents, explains Ben Jankovic at the University of New South Wales, Sydney.

Fitch and his colleagues propose that whistles enhance vocal clarity and transmission distance, although these theories require further investigation.