A horse’s whinny is a remarkably distinctive call that blends both high- and low-frequency sounds. In a study published in Current Biology, researchers explain how horses manage to produce unusually high-pitched tones despite their large body size, while also generating deeper sounds at the same time. They achieve this by creating a whistle within the larynx while simultaneously vibrating their vocal folds, much like humans do when singing. The team suggests that this dual sound production likely evolved to allow horses to communicate multiple types of information at once.
Elodie Briefer from the University of Copenhagen explains that the study clarifies how the two fundamental frequencies that form a whinny are generated. Previous research had shown that these two frequencies carry different emotional information about the caller. The new findings provide strong evidence that each frequency arises through a separate physical process.
Although domesticated horses have shared a close relationship with humans for more than 4,000 years, their vocal communication remains poorly understood. Typically, larger mammals produce lower-pitched sounds because larynx size generally increases with body size. Horses, however, are a notable exception to this pattern.
To investigate further, the researchers examined the mechanics behind the whinny and identified it as a case of biphonation — a vocal phenomenon in which a single call contains two independent frequency components, one low and one high. The lower frequency is produced through vocal-fold vibration, similar to human singing or a cat’s meow. Until now, the source of the higher frequency had remained unclear.
To solve this question, the team combined expertise from veterinary medicine and acoustic physics. They conducted detailed analyses of equine vocal anatomy, reviewed clinical data, and performed acoustic measurements. Their results revealed that the high-frequency component is created by a laryngeal whistle. This mechanism resembles a human whistle in principle, but in horses the turbulent airflow that generates the sound occurs inside the larynx.
While small rodents such as rats and mice are known to produce laryngeal whistles, horses are the first large mammals identified to use this mechanism. They are also the only known animals to produce a laryngeal whistle at the same time as vocal-fold vibration.
To confirm their findings, the researchers carried out experiments using larynges removed from deceased horses. They passed air through the excised larynges and alternated the airflow between normal air and helium. Because sound travels faster in helium, whistle frequencies increase under helium conditions, whereas frequencies produced by vibrating vocal folds remain stable. As expected, the high-frequency component rose when helium was used, while the lower frequencies stayed the same—clear evidence that the high tone results from a whistle.
William Tecumseh Fitch of the University of Vienna described the moment the frequency shift appeared under helium as immediate and decisive, confirming that the team had identified the correct mechanism.
These findings clarify how the two overlapping pitches in a whinny are produced and suggest that biphonation likely evolved to enable horses to transmit multiple independent signals simultaneously.
The researchers also observed that Przewalski’s horses, which are closely related to domesticated horses, produce biphonic whinnies as well. In contrast, more distant relatives such as donkeys and zebras appear to lack the high-frequency component. This suggests that horses possess unique vocal adaptations that allow for a richer and more complex range of calls than those of many other mammals.
David Reby of the University of Lyon/Saint-Etienne notes that understanding how and why biphonation evolved is a key step toward explaining the remarkable diversity of vocal behaviours found across mammal species.
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