How horses whinny has long been a mystery. The sound is quite distinct from any other in the animal kingdom. And now scientists think they’ve discovered why: horses whinny by producing sounds at two frequencies at the same time—much like singing and whistling simultaneously.
The findings, which were published on Monday in Current Biology, suggest horses produce sounds at two frequencies in two distinct ways in their larynx, or voice box. A low-frequency sound of about 200 hertz is produced by vibrating the vocal cords, just as we do while singing. And a high-frequency sound of more than 1,000 hertz is produced by whistling within the larynx.
“We now finally know how the two fundamental frequencies that make up a whinny are produced by horses,” said Élodie Briefer, an associate professor in the department of ecology and evolution at the University of Copenhagen and a co-author of the paper, in a statement.
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In general, the larger an animal—and its larynx—is, the lower the frequency of sound it produces. Meanwhile smaller critters, such as mice, produce high-frequency whistles. But this study suggests that horses are unique in their ability to whistle and vibrate their vocal cords at the same time, the authors write.
“This is going to be a landmark paper in terms of stimulating research into vocalizations in equids,” said Sue McDonnell, an adjunct professor of reproduction and behavior at the University of Pennsylvania, who was not involved in the research, to the New York Times.
The results support a theory that horses may have evolved to communicate multiple messages in a single vocalization, the study authors write.
“In the past, we found that these two frequencies are important for horses, as they convey different messages about the horses’ own emotions,” Briefer said in the same statement. “We now have compelling evidence that they are also produced through distinct mechanisms.”