Swiss researchers say they’ve decoded the vocalizations of horses enough to determine the nature and intensity of the emotions conveyed by specific whinnies.
Working at the Institute of Agricultural Sciences at ETH Zurich, the researchers analyzed the structure of different whinnies produced by 20 horses. Each whinny was triggered by specific situations designed to evoke positive or negative reactions of various intensities from the horses.
“We used social separation and reunion,” says Elodie Briefer, PhD. “We removed from the subject horse’s environment either one horse or all the other horses on the farm. These situations were assumed to be negative, as horses are social animals and would avoid such situations that would expose them to predators in the wild. We then brought the horses back toward the subject, which was assumed to be positive. The underlying valence (positive/negative) and intensity of the emotions were further verified using physiological and behavioral indicators. For instance, the intensity of the situation was determined
using the heart rate.”
Throughout the testing period, the researchers recorded vocalizations the study horses made and then analyzed the acoustical properties of each.
The data revealed that equine vocalizations, unlike those of most mammals, have two frequencies. “Most mammal vocalizations (and human voices, too) are composed of one fundamental frequency and its multiples, called ‘harmonics,’” says Briefer. “In horses, surprisingly, whinnies are always composed of two fundamental frequencies and their harmonics, that we conventionally named ‘G0,’ which is higher, and ‘F0,’ which is lower. This phenomenon is referred to as biphonation and is rare among mammals, especially with this pattern: All the whinnies are composed of these two frequencies.”
This phenomenon had gone undetected previously, says Briefer, because “nobody ever analyzed horse [vocalizations] in such detail---researchers usually ignored the second part of the whinny, where G0 and F0 overlap, which is actually easy to hear.”
Equine whinnies usually start with G0 and then F0 is added in, explains Briefer: “Humans and other mammals are able to sometimes produce two fundamentals when the pressure on the vocal cords is high, like during screaming. But it doesn’t happen constantly. In horses, we don’t know the mechanism of production, but it is probably through asynchronous vibration of the vocal folds.”
Another surprise came when the researchers compared the vocalizations the horses made during positive and negative situations. “Put simply, we found that F0 is a good indicator of emotional arousal [intensity] and G0 is [an indicator] of emotional valence,” says Briefer. “Valence and arousal are the two main dimensions of emotions: Valence is whether the emotion is positive, such as joy, or negative, such as fear. Arousal is whether the emotion is intense, like fear, or not, like depression. So when a positive or negative emotion becomes more intense, F0 increases. In a positive emotion, compared to a negative one, and independently from arousal, G0 becomes lower.”
Briefer says that handlers can learn to distinguish between whinnies. “Positive whinnies start with a lower frequency than negative whinnies. These whinnies are also much shorter than negative whinnies. I think it is quite easy to hear, once we know the trick---shorter and lower frequency whinnies are positive, while negative whinnies start much higher in frequency and are longer.”
Much more remains to be learned from analysis of equine vocalizations, says Briefer: “The next step for us is to find out if other horses can extract the emotional information and differentiate between negative and positive whinnies.”
Below are two audio recordings of different whinnies analyzed in this study, along with the researchers’ classification of each.
The audio files are composed of four whinnies produced by two different horses. The first two whinnies correspond to the first horse (negative and positive, respectively), and the next two whinnies to the second horse (negative and positive also).
Reference: “Segregation of information about emotional arousal and valence in horse whinnies,” Scientific Reports, April 2015
This article first appeared in EQUUS issue #454, July 2015.