A decade ago, biologist Elodie Briefer started recording horses’ whinnies. When she analyzed recordings of individual horses, she noticed two distinct, overlapping sounds: one with a low frequency, or pitch, and one with a high frequency. “I was quite confused,” says Briefer. “I thought I recorded two horses.” Now she’s finally figured out how horses create two tones at once.
Briefer’s team at the University of Copenhagen in Denmark inserted small cameras into horses’ noses—a common procedure that doesn’t hurt the animals. The researchers found that when air vibrates folds in a horse’s larynx, or voice box, it produces the lower-pitched sound. The horse’s throat also constricts. Air moving through this narrow passage produces a high-pitched whistle.
Scientists think horses’ two-toned whinnies allow them to pack more meaning into a single message. A complex whinny could let members of a herd know who’s calling, where they are at, and how they are feeling.