Perception of emotional valence in horse whinnies

Abstract

BackgroundNon-human animals often produce different types of vocalisations in negative and positive contexts (i.e. different valence), similar to humans, in which crying is associated with negative emotions and laughter is associated with positive ones. However, some types of vocalisations (e.g. contact calls, human speech) can be produced in both negative and positive contexts, and changes in valence are only accompanied by slight structural differences. Although such acoustically graded signals associated with opposite valence have been highlighted in some species, it is not known if conspecifics discriminate them, and if contagion of emotional valence occurs as a result. We tested whether domestic horses perceive, and are affected by, the emotional valence of whinnies produced by both familiar and unfamiliar conspecifics. We measured physiological and behavioural reactions to whinnies recorded during emotionally negative (social separation) and positive (social reunion) situations.ResultsWe show that horses perceive acoustic cues to both valence and familiarity present in whinnies. They reacted differently (respiration rate, head movements, height of the head and latency to respond) to separation and reunion whinnies when produced by familiar, but not unfamiliar individuals. They were also more emotionally aroused (shorter inter-pulse intervals and higher locomotion) when hearing unfamiliar compared to familiar whinnies. In addition, the acoustic parameters of separation and reunion whinnies affected the physiology and behaviour of conspecifics in a continuous way. However, we did not find clear evidence for contagion of emotional valence.ConclusionsHorses are thus able to perceive changes linked to emotional valence within a given vocalisation type, similar to perception of affective prosody in humans. Whinnies produced in either separation or reunion situations seem to constitute acoustically graded variants with distinct functions, enabling horses to increase their apparent vocal repertoire size.Electronic supplementary materialThe online version of this article (doi:10.1186/s12983-017-0193-1) contains supplementary material, which is available to authorized users.

Vocal contagion of emotions in non-human animals

“…This is what we attempted in horses, by playing back, as described in the Discrimination of emotional valence section, whinnies produced during social reunion ( positive) and separation (negative). However, we did not find clear evidence for state matching between emitters and receivers, because horses did not display more behaviours indicating negative emotions (head high [8]) during playbacks of negative whinnies, nor more behaviours suggesting positive emotions (chewing motion [8]) during playbacks of positive whinnies [29]. Similar tests conducted in other species would thus be useful to show if contagion of valence can indeed occur within a given vocalization type and thus result from the information about emotional valence conveyed in these calls, more than their function.…”

“…Evidence for discrimination of vocal expression of emotional valence is sparse. My colleagues and I have tested the ability of horses (Equus caballus) to differentiate between positive and negative whinnies by directly playing back these two whinny variants separately (without habituation) [29]. This allowed us to investigate, simultaneously, discrimination and contagion of valence (see the Contagion of emotional valence section for the results regarding contagion).…”

Indicators of Horse Welfare: State-of-the-Art

“…Horses' acoustic communication has not been thoroughly investigated, and although whinnies are known to encode both social [123] and emotional information [142], information carried by non-vocal communication (snore, snort, blows) are less clear. Because descriptions are imprecise, snorts and snores have been confused for a long time, both were considered as indicative of high emotional states [143,144].…”

Emotional Transfer in Human–Horse Interaction: New Perspectives on Equine Assisted Interventions

“…A signal performed by an individual could induce in the receiver both the same arousal level (i.e., contagion of emotional arousal) and the valence (i.e., contagion of emotional valence). Briefer et al [106] evaluated if horses are able to decode the emotional valence and arousal of whinnies performed by familiar or unfamiliar conspecifics and whether any form of emotional contagion occurs. Results showed no clear evidence of contagion of emotional valence; nonetheless, authors demonstrated that horses reacted differently to separation and reunion whinnies when they are produced by familiar conspecifics, but no differences were found when unfamiliar individuals performed them, thus advocating the occurrence of emotional arousal transfer.…”