Horses show impressive ability to recognise emotional tone in study

Domestic horses show ability to recognise emotional tone in their own kind, humans, and even Przewalski's horses.
Image by JackieLou DL

Domestic horses seemed able to recognise the negative emotional tone of recordings from not only other horses in a study, but from Przewalski’s horses and humans.

Przewalski’s horses were also able to recognise the negative tone in vocalisations from not only their own kind, but domestic horses and humans.

Pigs showed a similar ability, recognising the negative emotional tone from other pigs as well as wild boars and humans.

Researchers Anne-Laure Maigrot, Edna Hillmann and Elodie Briefer, writing in the journal BMC Biology, said discrimination and perception of emotion expression regulate interactions within a species. They can lead to emotional contagion – in which the state between the producer and receiver are matched – or to more complex forms of empathy, such as sympathetic concern.

Empathy processes are enhanced by familiarity and physical similarity between partners, they said.

Since animals from different species can also be familiar with each other to some extent, discrimination/perception of emotions and, as a result, emotional contagion could also occur between species.

The trio, with ETH Zürich, a public research university in Switzerland, set out to investigate if four species – domestic horses, Przewalski’s horses, pigs and wild boars – can discriminate between vocalizations of opposite emotional valence (positive or negative), produced not only by their own kind, but also by closely related species and humans.

With this aim, they played back to individuals of these four species, which were all used to humans, vocalizations from a set of recordings for which the emotional valence was known.

All species heard positive and negative vocalisations from their own kind and from humans. Vocalisations from domestic horses were played to Przewalski’s horses and vice versa; and vocalisations from pigs were played to wild boars and vice versa.

“We found that domestic and Przewalski’s horses, as well as pigs, but not wild boars, reacted more strongly when the first vocalization played was negative compared to positive, regardless of the species broadcasted,” they reported.

Both the domestic horses and the Przewalski’s horses spent more time walking and less time standing to the playbacks when the first vocalizations were negative compared to positive, regardless of whether these vocalizations were produced by their own kind, the closely related species, or humans.

In addition, the Przewalski’s horses showed signs of more attentiveness when they first heard Przewalski negative calls than positive calls. This was not the case when they heard domestic horse calls or human voices.

Domestic pigs likewise reacted more strongly to the playbacks when the first ones played were negative compared to positive, including those from boars.

By contrast, wild boars did not react differently to positive and negative vocalizations of wild boars or humans, but they did react to the positive and negative calls of domestic pigs. “They moved their head more often, spent more time with their ears on the sides, with the tail high and standing, and produced more grunts, when positive pig calls were played compared to negative ones.”

For all species, the response to the human voice was less marked. For example, domestic horses were slower to respond when hearing human voices compared to their own kind.

The researchers said the potential factors at play in their findings were familiarity with the species, domestication, and evolutionary links (phylogeny).

The familiarity hypothesis predicts that animals should distinguish emotional tone better in familiar than unfamiliar species, after learning from repeated exposure. This hypothesis did not seem to be supported by the results, they said.

The domestication hypothesis predicts that domestic animals should discriminate/perceive human emotional states better than wild species, following a selection (likely unconscious) during domestication, of those with this ability.

“Accordingly, our results suggest that both domestic species (horses and pigs) are able to discriminate between positive and negative human meaningless speech. Also in accordance with this hypothesis, wild boars do not seem able to do so.

“However, our results suggest that Przewalski’s horses, which have not been domesticated, or only briefly, are able to discriminate between human vocalizations of opposite valence as well.”

Therefore, the domestication hypothesis could explain the results obtained in pigs and boars, but not in Equidae.

The phylogeny hypothesis predicts that, due to conservation of indicators of emotions throughout evolution, animals should discriminate/perceive the valence encoded in the vocalizations of their own kind better, or at least as well, as in those of closely related species.

The results suggest that, in line with this hypothesis, domestic horses, Przewalski’s horses, and pigs are able to discriminate between positive and negative vocalizations of their own kind and a closely related species, but also human speech.

Finally, all species reacted as strongly to the calls of the closely related species as to the calls of their own kind, while they displayed weaker responses to humans.

“This suggests that the acoustic structure of domestic horse and Przewalski’s horse whinnies, as well as of domestic pig and wild boar grunts, is similar enough to trigger a species-specific response,” they said.

Indeed, it has been shown that the structure of Przewalski’s horse whinnies resembles the structure of domestic horse whinnies. Pigs and wild boars also produce grunts that are similar in structure.

“To conclude, domestic horses, Przewalski’s horses, and pigs were found to distinguish vocal indicators of valence in all the species we played back, while wild boars only did so in pig calls.

The results obtained in Equidae could thus be explained by the phylogeny (evolutionary) hypothesis, while the responses of pigs and wild boars were more in line with the domestication hypothesis.

Further studies could investigate if these results hold when considering more species and families, and also other channels, such as sight and smell.

“Our results also suggest that the valence of the human voice can have an impact on the emotional states of domestic and captive animals, and stress the need to further assess if, how, and when does human-animal vocal emotional contagion occur, using additional indicators of emotions not used in this study (for example, physiological or cognitive indicators).”

Maigrot, AL., Hillmann, E. & Briefer, E.F. Cross-species discrimination of vocal expression of emotional valence by Equidae and Suidae. BMC Biol 20, 106 (2022).

The study, published under a Creative Commons License, can be read here

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