A laryngeal whistle revealed
In a paper just published in Current Biology, Lefèvre and colleagues have resolved this mystery. The high-frequency component is actually generated by a laryngeal whistle. This is similar in principle to a normal human lip whistle, except that the turbulent airstream is created within the horse's larynx (voice box). The scientists demonstrated this by obtaining both low- and high-frequency components from the larynx alone (the horse larynges were obtained from the food industry).
To prove that the high component is a whistle, the researchers switched the airstream into the larynx from air to helium and back again. Because the speed of sound is higher in helium, it causes whistles to shift to higher frequencies, while the frequency of normal vocal fold vibration remains unchanged. As predicted, the high-frequency component of the whinnies shifted upward in helium, while the low frequencies did not change.
Prof. Tecumseh Fitch of the University of Vienna, who co-led the study with Elodie Briefer (University of Copenhagen), has been studying mammal vocal production for decades but he said it was the first time he had seen such a laryngeal whistle himself. "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. We were thrilled!"
New insights into mammal vocal capabilities
The study, which took years to complete, also provided converging anatomical, endoscopic, and clinical data that support their claim. Lead author Dr. Romain Lefevre, who conducted this research as part of his PhD at the University of Copenhagen said: "Solving this biomechanical puzzle required combining approaches from veterinary medicine to acoustic physics. Proving that the high frequency in whinnies is actually a whistle (generated by air turbulence rather than tissue vibration) forces us to reconsider the vocal capabilities of large mammals."
His PhD supervisor, Associate Prof. Elodie Briefer said "We now finally know how the two fundamental frequencies that I first described in 2015 are produced by horses. In this original study, we had found that these two frequencies are important for horses, as they convey different messages about the horses’ own emotions. We now have compelling evidence, using methods spanning from endoscopies to excised larynges, that they are also produced through distinct mechanisms."
Although some small rodents, like rats and mice, produce their ultrasonic calls using laryngeal whistles, horses are the first large mammal species where this has been documented. Furthermore, rats and mice do not combine this whistle with a lower frequency simultaneously produced by vocal fold vibration. Intriguingly, Przewalski's horses (which are closely related to the domesticated horse) also produce whinnies with biphonation, but more distant horse relatives like donkeys and zebras appear to lack the high component.
Thus, this appears to be a uniquely "horsy" adaptation, giving horse vocalizations a richer and more complex call spectrum than seen in most other mammals.
Outside commentaries:
Coen Elemans, Christian Herbst, Karen McComb
Contact:
Prof. Tecumseh Fitch
+43 676 343 5572
