Swiss researchers examining the whinnies of horses found evidence suggesting the presence of different underlying emotions in their vocalisations.
They also discovered that all whinnies contained two independent fundamental frequencies, which they labelled F0 and G0.
They found that F0 element and whinny energy encoded emotional arousal, such as whether the animal was calm or excited, while G0 and the whinny duration encoded what they termed valence – whether the emotion was negative or postiive.
Elodie Briefer and her colleagues said the cues to emotional arousal and valence in horse whinnies were segregated in different, relatively independent parameters.
“Most of the emotion-related changes to vocalizations that we observed are similar to those observed in humans and other species, suggesting that vocal expression of emotions has been conserved throughout evolution,” they wrote in Scientific Reports.
The use of the two fundamental frequencies allowed horses to express both positive and negative emotions, and at the same time convey the strength of these emotions.
“One frequency indicates whether the emotion is positive or negative, while the other frequency reveals the strength of the emotion,” explains Briefer.
The two frequencies have not been described in any previous study on horse vocalisations until now, according to the researchers, 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,” she says.
Click to hear an audio sample of two horses that express, successively, negative and positive emotions through their whinnies.
It is not yet known how horses simultaneously produce such complex sounds. The researchers suspect it is due to an asynchronous vibration pattern of the vocal cords.
Briefer, who is in the Ethology and Animal Welfare Unit at ETH Zurich’s Institute of Agricultural Sciences, and her colleagues set about testing 20 privately owned horses of various breeds and age, housed in five different farms, in groups of three to five.
The horses were exposed to various positive and negative situations, involved around separation or reunion with their equine companions. This allowed the researchers to study an individual horse’s reaction when members of the group were removed and then later returned.
They 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.
Their findings showed that the intensity of emotions was 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 – 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 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.
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.
The research is part of a larger research project that is exploring how the expression of emotions has evolved among various hooved animals, in particular the effects arising from 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’s horses, domestic pigs and wild boars, and cattle and bison.
Whinnies are the longest, loudest and most common horse vocalization. Three parts have previously been described:
- The “introduction”, which is tonal and high in frequency;
- The “climax”, which is a long, often frequency and amplitude modulated part;
- The “end”, which is low in frequency and amplitude, and composed of a pulse-train structure.
However, their discovery of the two independent fundamental frequenies in all whinnies – a phenomenon known as biphonation, which is rare in mammals – resulted in the researchers carrying out detailed vocal analysis to rule out alternative explanations.
They confirmed that the two frequencies did not simply result from a register change, finding that they actually overlapped. They also verified that they were not harmonically related
“Our results show that F0 and G0 are not generated by artifacts, nor are they harmonically related, suggesting real biphonation in whinnies.
“G0 was present, in addition to F0, in all whinnies, indicating that the presence of two fundamental frequencies is a common feature of this type of call.”
The research team concluded that the elements of the whinny could allow individuals to generate “highly complex and unpredictable vocalizations”.
“Our results show that the presence of two fundamental frequencies can also function as a means of emotion expression, with each frequency encoding one emotional dimension (i.e. arousal and valence).”
The evidence, they said, suggested that the emotional state of horses can both be effectively and simultaneously communicated in horses.
Briefer was joined in the research by Anne-Laure Maigrot, Roi Mandel, Sabrina Briefer Freymond, Iris Bachmann and Edna Hillmann.
Briefer EF, Maigrot AL, Roi Mandel R, Briefer Freymond S, Bachmann I, Hillmann E: Segregation of Information about Emotional Arousal and Valence in Horse Whinnies. Scientific Reports, 21. April 2015, DOI: 10.1038/srep09989
The full study can be read here.