Horses whinny in two voices, which let them express both positive and negative emotions, as well as the strength of that feeling.
The findings come from a research project on the evolution of emotion expression.
Each whinny is made up of two independent fundamental frequencies, according to researchers at the ethology and animal welfare unit at ETH Zurich’s Institute of Agricultural Science.
“One frequency indicates whether the emotion is positive or negative, while the other frequency reveals the strength of the emotion,” explains Elodie Briefer, the project leader.
Listen: A horse’s whinny starts with a high frequency tone, followed by a lower one.
Listen: two horses, which express successively negative and positive emotions through their whinnies.
This phenomenon had not been described in any previous study on horse vocalizations, even though listeners with normal hearing can easily perceive both fundamental frequencies if they are aware of them, according to Briefer.
“Such vocalizations with two fundamental frequencies are rare among mammals, in contrast, for example, to songbirds,” Briefer explains. It is not yet known how horses simultaneously produce such complex sounds. Researchers suspect that the presence of these two fundamental frequencies is due to an asynchronous vibration pattern of the vocal cords.
In order to learn more about the expression of emotions in horses, researchers tested 20 groups of horses by exposing them to various positive and negative situations. This allowed researchers to study an individual horse’s reaction when members of the group were removed and then later returned.
Researchers used cameras and microphones to record the behavior and vocalizations of the horses and also measured the animals’ physiological responses, such as heart rate, breathing, and skin temperature.
Higher or lower?
The heart and respiratory rates, the horses’ movements, the characteristics of the lower of the two fundamental frequencies of the whinny and the amplitude of higher frequencies best indicate the intensity of emotions.
Specifically, the more aroused the horse is, the more its heart rate and breathing increase. It moves more and produces whinnies in which the lower of the two fundamental frequencies is higher, regardless of whether the emotion is positive or negative.
The valence—that is, whether the emotion is positive or negative—is expressed most strongly through the characteristics of the duration of the whinny, the higher fundamental frequency, and the position of the head.
Positive emotions are recognizable when a horse emits whinnies of shorter duration and in which the higher fundamental frequency is lower, and it lowers its head. Whinnies produced during negative emotions are longer and the higher fundamental frequency is higher.
This knowledge could be useful to both horse-owners and veterinarians, allowing them to better interpret the animal’s behavior and thus respond more effectively to its needs.
This research, published in Scientific Reports, is part of a larger project exploring how the expression of emotions has evolved among various ungulates. The researchers want to find out whether domestic animals and their wild counterparts express their emotions in a similar way, or if domestic species have adapted their means of expression to humans.
Comparisons are planned between domestic and Przewalski horses (a species of wild horse), domestic pigs and wild boars, and cattle and bison.
Source: ETH Zurich