We already know that horses communicate a great deal of information through their ears and eyes. Now, new research is showing that equines convey both positive and negative emotions to one another through their complex vocalizations.

Scientists at the Ethology and Animal Welfare Unit at ETH Zurich’s Institute of Agricultural Science discovered that every whinny contains two independent frequencies, each communicating different information about a horse’s emotions.

“One frequency indicates whether the emotion is positive or negative, while the other frequency reveals the strength of the emotion,” said project leader Elodie Briefer. “Such vocalisations with two fundamental frequencies are rare among mammals, in contrast, for example, to songbirds.”

To obtain these findings, the researchers tested 20 groups of horses by putting them in a variety of positive and negative situations. Using cameras and microphones, the scientists recorded the reactions of each horse when one was removed from the group and then brought back. They also measured each equine’s heart rate, breathing and skin temperature. If you’re looking to "speak horse" — to identify positive or negative horse vocalizations — this kind of information will help you decode the sounds.

Through these tests the researchers discovered that positive emotions were accompanied by shorter whinnies. The higher frequency in those shorter whinnies was lower and the horse also lowered its head. When a negative emotion was being conveyed, the whinny was longer and the higher fundamental frequency was higher.

Beyond knowing if an emotion was negative or positive, researchers were able to gauge the intensity of each feeling. By looking at factors like the horses’ respiratory rates, physical movements, and the higher and lower frequencies of the horses’ whinnies, researchers were able to see the intensity of the emotion a horse was feeling at that time. For example, the more aroused the individual was, the higher the heart rate and the bigger the increase in breathing. The horse’s lower frequency was also higher whether or not the emotion that horse experienced was positive or negative.

As for how horses are able to make these two fundamental frequencies, researchers are still in the dark. They hypothesize that they are produced through an asynchronous vibration pattern of the vocal cords.

Anyone who has spent time around horses knows that a whinny can range from ear piercingly high-pitched to a low calming rumble. And while sometimes it’s obvious what a horse is feeling based on the situation, other times humans are baffled by some vocalizations and body language displays. The researchers in Switzerland believe that this new information can be useful to veterinarians and horse owners, enabling them to better understand a horse’s behavior, and therefore be in a better position to meet the individual’s needs.

The study is a part of a larger project looking into the effect of domestication. Scientists are interested in finding out how domestic animals and their wild relatives express emotions, whether or not those expressions are different or similar, and if domestic animals have altered their methods of communicating due to their interactions with humans. They plan to compare domestic horses with Przewalski horses, domestic pigs with wild boars, and cattle with bison.

Related on MNN: