How horses express emotion

A whinny is not just a whinny. Horses can convey complex information when they whinny. These expressions reflect their emotions, in the same way as human¡¯s voice. 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.

Download Audio sample (WAV, 605 KB): A horse¡¯s whinny starts with a high frequency tone, followed by a lower one.

This phenomenon had not been described in any previous study on horse vocalisations until now, despite the fact that listeners with normal hearing can easily perceive both fundamental frequencies if they are aware of it, according to Briefer. ¡°Such vocalisations 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 behaviour and vocalisations of the horses and also measured the animals¡¯ physiological responses, such as heart rate, breathing and skin temperature.

The findings show that the intensity of emotions is best indicated by 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. 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 ¨C that is, whether the emotion is positive or negative ¨C 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 can be recognised by the fact that the 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.

Download Audio sample (WAV, 1.1 MB) of two horses, which express successively negative and positive emotions through their whinnies.

This knowledge could be useful to both horse owners and veterinarians, allowing them to better interpret the animal¡¯s behaviour and thus respond more effectively to its needs. This research is part of a larger research project that explores how the expression of emotions has evolved among various ungulates. The main aim of this project is to look at the effect of domestication. 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.