Whinnies seem to play a role in horses’ social interactions. Credit: Mumemories/Getty
The horse whinny, or neigh, has been a familiar sound at least since the animal was domesticated, around 4,200 years ago. But until now, scientists didn’t understand how horses produced this characteristic vocalization.
New research shows that horses (Equus ferus caballus) whinny by making two distinct sounds at once: a low-frequency ‘nickering’ sound made in their vocal folds, combined with a high-pitched whistle produced by forcing air through the cartilage of their larynx. The biomechanics of this dual-frequency sound, or biphonation, are described on 23 February in Current Biology1. The authors theorize that this multilayered vocalization enables the animals to convey complex messages to each other.
The animals’ ability to whistle while vocalizing earns them membership into a small club of creatures that can make biphonic sounds, which includes several birds species, human beat boxers, Mongolian throat singers, and those who can sing while whistling.
Two-toned neighs
Of all the noises horses make — grunts, squeals, snorts and more — the whinny is the most common, and the longest in duration. Being social animals, horses whinny to greet other horses, to keep in contact with each other across long distances, and in fear or stress — in the wild, it can be dangerous for them to get separated from the herd. “Each horse has a different whinny,” says Elodie Briefer, a behavioural ecologist who studies animal communication at the University of Copenhagen, Denmark.
About ten years ago, Briefer published evidence2 that the horse whinny is made up of two distinct fundamental frequencies, one a high whistling sound at more than 1,000 Hz, and the other a low one at about 200 Hz. Her work also showed how differences in both parts of the sound correspond to positive or negative emotions, and the intensity of those emotions. But, she says, it took years to figure out how horses produce these complex sounds.
Her team started by imaging horses’ larynges using endoscopy, a camera-technique which veterinarians commonly used diagnose breathing problems in the animals. They recorded footage of ten stallions, including when they were introduced to mares. In these videos, Briefer says, they could see that the animals’ vocal folds vibrated, as would be expected, and also that the cartilage above the stallions’ vocal folds contracted.
Next, they obtained horse larynges from butchers in France, where the animals are used for meat, and mimicked the whinny ex vivo with the help of University of Vienna bioacoustics expert Tecumseh Fitch.
The experiments showed that the high-frequency part of the call is an acoustic whistle, a noise caused not by vibrating tissue but by vibrations of air moving through a tube. (High-pitched noises made by other mammals, including dolphins, are colloquially called whistles, but are technically not true whistles.) The researchers discovered this using helium. Because the speed of sound varies in different types of gas, changing the composition of air blown through the larynx should change the pitch of a whistle. When the researchers added helium to the air, the frequency of the higher pitch increased, and that of the low pitch stayed the same. This proves that the high-frequency sound is a whistle, whereas the low one is caused by vibration of the vocal folds, the researchers conclude.
