Emotional contagion of fear and joy from humans to horses using a combination of facial and vocal cues.

Emotional contagion, the emotional state-matching of two individuals, has been documented in various species. Recent findings suggest emotional contagion could also take place between humans and domestic mammals. However, the range of targeted animal species and human emotions that have been studied is still limited, and the methodology to investigate emotional contagion in this context is not fully established. This study examined emotional contagion of fear and joy from humans to horses by measuring physiological (heart rate, infrared thermography) and behavioral responses (posture, laterality bias, facial expressions) to videos of human fear, joy, or neutral emotions. Horses (n = 45) exhibited higher heart rates and ear movements during the fear and joy videos compared to the neutral ones, suggesting heightened arousal. During fear videos, they showed a greater increase in eye temperature than during joy or neutral videos, and maintained an alert posture for longer than during neutral videos, expressing specific facial expressions including inner brow raising and blowing. During joy videos, they showed a higher right eye preference (indicating a higher left-hemisphere bias) than during neutral videos, suggesting a more positive emotional valence. These findings suggest that human facial and vocal cues of fear and joy trigger emotional contagion in horses, which may influence the human-horse relationship.