Researchers have discovered that horses generate a low-pitched moo-like sound and a high-pitched whistle at the same time using their larynx, a vocal ability unlike any other large animal. This biphonation involves vibrations in the vocal folds for the low frequency and an aerodynamic whistle for the high one. The finding sheds light on equine communication after millennia of human-horse interaction.
Horses whinny by creating two distinct sounds simultaneously: a low-frequency component around 200 hertz, similar to a cow's moo, and a high-frequency one above 1000 hertz, resembling a whistle. This phenomenon, known as biphonation, has puzzled scientists because the high pitch is unusual for such a large mammal.
Tecumseh Fitch at the University of Vienna, Austria, and his team investigated by conducting tests on horse larynges sourced from a meat supplier. They blew air through the larynges and initially produced only the low-frequency sound. "But with some playing around, we were able to obtain the high-frequency component as well," Fitch explained.
To confirm the mechanism, the researchers used both air and helium, gases with different densities. "The low-frequency component is produced by vocal fold vibration, just like human singing or a cow mooing, and it didn’t shift when we changed between gases," Fitch noted. In contrast, "the high-frequency component does significantly shift to be higher in helium, just as predicted for whistles."
Endoscopic examinations of live horses revealed that at the start of a whinny, laryngeal muscles contract, narrowing the glottis and tilting the vocal folds. This increases airway resistance, forcing air through a slit-like opening at high speed to create the whistle.
While mice and rats can produce laryngeal whistles, theirs are at frequencies inaudible to humans. "Horses are the only mammal known to use the larynx to produce two frequencies simultaneously where one is a whistle and, in fact, they’re the only large mammal, besides humans, to produce whistles as part of the standard vocal repertoire," Fitch stated.
Ben Jancovich at the University of New South Wales in Sydney, Australia, described the work as "the first strong, experimental evidence of an aerodynamic laryngeal whistle production in any animal outside the rodent family."
Fitch highlighted the gap in understanding: "Although humans have been co-existing and co-evolving with horses for 4000 years, we still understand their communication imperfectly."
The team suggests the whistle might enhance the whinny's noticeability and range, though this remains untested. The study appears in Current Biology (DOI: 10.1016/j.cub.2026.01.004).
