Human emotional chemical cues collected in situations of disgust or sadness modulate horses’ behaviour and physiology in a multimodal context

Abstract Human emotional chemical cues influence the behaviour and physiology of several domestic species, including dogs and horses. While previous studies have mainly focused on fear and happiness, less is known about sadness and disgust, which may convey distinct affective information. This study examined whether human chemical cues collected during sadness and disgust -two negatively valenced emotions differing in arousal and presumed adaptive function - could modulate horses’ behavioural and physiological responses when presented with corresponding visual and auditory cues. Twenty-eight Welsh mares were exposed, while viewing emotion-matched facial expressions, to sweat samples collected from human donors during either neutral or emotional inductions (sadness or disgust). The two groups were tested twice, once with neutral chemical cues and once with emotional chemical cues. Compared with the neutral condition, emotional chemical cues altered horses’ physiological and behavioural responses: sadness-related cues induced lower physiological arousal and calmer postures, whereas disgust-related cues elicited increased heart rate and movement. Despite their shared negative valence, sadness and disgust elicited opposite behavioural and physiological responses consistent with their typical arousal levels. These findings suggest that horses can discriminate between emotion-specific chemosensory cues. By providing the first evidence of horses’ responses to human sadness- and disgust-related chemical cues, this study extends previous work beyond fear and highlights the role of olfactory cues in multimodal processing of emotional information across species.