Horses whinny to find new friends, greet old ones and mark happy moments such as feeding time, but scientists have had few answers about how they generate the distinctive sound. In a study published Monday in the journal Current Biology, researchers report that the horse’s whinny is not a single noise but a combination of low- and high-pitched tones created through different processes in the voice box.
The low-pitched part, the researchers said, is driven by air moving over bands of tissue in the voice box that vibrate—an approach they described as similar to how humans speak and sing. The high-pitched portion, by contrast, has been more puzzling for scientists because, with some exceptions, larger animals typically produce lower sounds, not whistles.
To investigate what the high tones are made of, the researchers used a small camera delivered through horses’ noses to film activity inside while the animals whinnied and also made the softer, subtler nicker. They also carried out detailed scans and blew air through isolated voice boxes taken from dead horses, the study said, to test how sound could be generated from the anatomy itself.
The researchers concluded that the high-pitched component is a kind of whistling that begins in the voice box. In their description, vibrating tissue produces the whistle as an area just above contracts, leaving a small opening for the whistling sound to escape. They contrasted that with human whistling, which they said is produced differently because people whistle through their mouths.
Jenifer Nadeau, a horse researcher at the University of Connecticut who was not involved in the study, said, “I’d never imagined that there was a whistling component. It’s really interesting, and I can hear that now.” Another equine-science expert, Alisa Herbst with Rutgers University’s Equine Science Center, said the findings were exciting because the whinny includes two different fundamental frequencies generated by two different mechanisms.
The study also placed the findings in a broader biological context by noting that certain small rodents such as rats and mice whistle this way, while horses are, so far, the first known large mammal with the knack for it. The researchers also said horses are the only animals known to whistle through their voice boxes while they sing.
The remaining question for scientists, the report said, is how horses evolved a two-toned call that combines those distinct sound components. It said wild Przewalski’s horses can do something similar, as can elks, but more distant relatives such as donkeys and zebras do not make the high-pitched sounds.
Study author Elodie Mandel-Briefer of the University of Copenhagen said the two-toned whinnies could help horses convey multiple messages at the same time. She said, “They can express emotions in these two dimensions,” suggesting the different pitches may support a more complex range of feelings during social interactions.