Horses communicate using a distinctive vocalization – the whinny – which is unique among large mammals due to its simultaneous production of both low and high-frequency sounds. For centuries, the precise mechanism behind this “biphonation” has remained a mystery. New research reveals that horses generate these sounds not just through vocal cord vibration (like mooing or human speech) but through a specialized aerodynamic whistle produced directly within the larynx.
The Mystery of Biphonation Explained
A horse’s whinny contains two distinct acoustic components: a low-frequency rumble around 200 hertz and a high-frequency whistle exceeding 1000 hertz. The low frequency is easily explained by the movement of vocal folds in the larynx, similar to how cows moo. The high-frequency whistle, however, baffled scientists because it is unusual for large animals and no other mammal is known to produce this effect in a similar way.
How Researchers Cracked the Code
Tecumseh Fitch at the University of Vienna and colleagues solved the puzzle through a series of experiments on horse larynges (obtained from a meat supplier). Initial tests only produced the low-frequency rumble, but further manipulation revealed how to generate the high-frequency whistle.
The team then tested whether the whistle was produced by the larynx itself or by the lips (as with human whistling) by blowing air and helium through the larynges. The low frequency remained consistent regardless of the gas used, while the high frequency shifted upwards in helium, confirming that it was created aerodynamically within the larynx.
The Mechanism: Muscles, Airflow, and Resistance
Endoscopic tests on live horses revealed that the whinny begins with muscle contractions around the larynx, narrowing the glottis (the opening between vocal folds). This constriction forces air through a narrow slit at high speed, creating a whistle.
Researchers found that rodents (mice and rats) also produce laryngeal whistles but at frequencies too high for human hearing. Horses are the only large mammal known to regularly use this mechanism; humans are the only other mammal that produces whistles as part of their natural vocalizations.
Implications and Future Research
The discovery provides strong experimental evidence of aerodynamic whistle production in a non-rodent animal. While the exact purpose of the whistle remains unknown, scientists speculate that it may increase the whinny’s range or audibility. Further research is needed to confirm whether the dual-frequency sound carries farther or is otherwise more effective than a single-frequency call.
This work sheds light on a previously misunderstood aspect of horse communication, demonstrating the remarkable complexity of their vocal repertoire and providing a deeper understanding of how mammals use the larynx for sound production.
