As a horse owner, you’re probably pretty familiar with the full range of sounds your horse emits: you know when he is shouting for food, you know the noise your mare makes when she is showing off to geldings and you know that low, quiet nickering when your horse first sees you. We have long known that our horses “talk” to us, but until now it wasn’t something that was scientifically codified. That was until a team of Swiss researchers identified the emotions conveyed by specific types of equine vocalizations. So what, precisely, did they find out?
Researchers at the Institute of Agricultural Sciences at ETH Zurich believe they’ve decoded the vocalizations of horses enough to determine the nature and intensity of the emotions conveyed by specific whinnies and utterances.
Using positive and negative reactions and studying the intensity of the whinnies, the team included social separation and reunion as triggers for reactions. In one test, all the neighboring horses were removed from the “subject” horse’s barn. These situations were assumed to be negative, as horses are extremely social animals and would be desperate to avoid being isolated in the wild. The horse’s neighbors were then brought back into relative contact with the subject horse, 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 subject horse’s reaction to the situation was determined by measuring vital signs such as heart rate and blood pressure.
For each reaction, the researchers recorded vocalizations made by the study horses and then analyzed the acoustical properties of each.
The study results demonstrated that equine vocalizations, unlike those of most mammals, have two frequencies. Most mammals – including humans – have vocalizations that are composed of one fundamental frequency and its multiples, called “harmonics.” However, whinnies from horses comprise of two fundamental frequencies and their respective harmonics, one higher and one lower. This is extremely rare in mammals and is known as biphonation.
This was the first time that the acoustic composition of whinnies had been analyzed to this extent. The team discovered that a whinny usually starts with the higher frequency and the lower frequency is then added in. It is possible for humans to produce mixed frequencies during high pressure on the vocal cords (screaming) but this cannot be sustained. Therefore it is of interest as to how horses are able to produce such sounds and the mechanisms behind it. While not yet completely understood, this is thought to be done through asynchronous vibration of the vocal folds.
Delving further into the structure of the equine vocalizations, additional differences were noted in horses during positive and negative situations. The researchers noted a pattern where the lower range frequencies were an indicator of emotional arousal and the higher range an indicator of emotional valance. 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, the range of whinny increases. In a positive emotion, compared to a negative one, and independently from arousal, the range of whinnies become lower.
You will intuitively know the tones your horse makes when he whinnies. But with this information, you can learn to interpret them further. In simple terms, positive whinnies start with a lower frequency than negative whinnies. These whinnies are also much shorter than negative whinnies.
This study is a great starting point for beginning to understand how horses communicate through vocalization. The research team who undertook this study will next be looking into the horse’s ability to interpret emotional context and differentiate between negative and positive whinnies. We look forward to understanding more!
Image courtesy of Dollar Photo Club
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