Domestic horses (Equus caballus) discriminate between negative and positive human nonverbal vocalisations

Amy Victoria Smith,Karen Mccomb,Kate Grounds,Leanne Proops,Sophie K Scott,Jennifer Wathan

doi:10.1038/s41598-018-30777-z

Amy Victoria Smith, Karen Mccomb + Show 4 more

Open Access

https://doi.org/10.1038/s41598-018-30777-z

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Abstract

The ability to discriminate between emotion in vocal signals is highly adaptive in social species. It may also be adaptive for domestic species to distinguish such signals in humans. Here we present a playback study investigating whether horses spontaneously respond in a functionally relevant way towards positive and negative emotion in human nonverbal vocalisations. We presented horses with positively- and negatively-valenced human vocalisations (laughter and growling, respectively) in the absence of all other emotional cues. Horses were found to adopt a freeze posture for significantly longer immediately after hearing negative versus positive human vocalisations, suggesting that negative voices promote vigilance behaviours and may therefore be perceived as more threatening. In support of this interpretation, horses held their ears forwards for longer and performed fewer ear movements in response to negative voices, which further suggest increased vigilance. In addition, horses showed a right-ear/left-hemisphere bias when attending to positive compared with negative voices, suggesting that horses perceive laughter as more positive than growling. These findings raise interesting questions about the potential for universal discrimination of vocal affect and the role of lifetime learning versus other factors in interspecific communication.

Highlights

The production and discrimination of emotional signals is a highly significant component of social living in mammals, as this allows for the efficient transmission of social intentions and the sharing of environmental information[1,2]
The facial expression associated with the affective state can influence the sound, through
Horses are sensitive to cues of affective state in conspecific vocalisations[17] and may be predisposed to attend to emotional cues embedded in vocalisations generally

Summary

Introduction

The production and discrimination of emotional signals is a highly significant component of social living in mammals, as this allows for the efficient transmission of social intentions and the sharing of environmental information[1,2]. Horses are sensitive to cues of affective state in conspecific vocalisations[17] (see also18) and may be predisposed to attend to emotional cues embedded in vocalisations generally They have been shown to discriminate socially relevant cues in human voices, such as voice identity characteristics during individual recognition[19]. Horses can distinguish human emotional states through other modalities such as through facial expression[20], and are sensitive to changes in human anxiety levels[21] As humans use their voices extensively during direct interaction with horses in riding, training, and groundwork it is likely that horses would benefit from discriminating between different emotions expressed in human voices, as this would allow them to better predict the consequences of their interactions with humans

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