Investigating differing auditory stimuli on heart rate and micro facial expressions of horses to promote positive affective states – A preliminary behavioural study. 

This study was completed as part of Megan Hutchinson’s studies for a BSc (Hons) degree in Animal Science at Dutchy College in collaboration with The University of Plymouth.

Modern equine management practices may impose constraints that conflict with horses’ evolutionary adaptations, potentially inducing stress that negatively impacts both health and welfare. Increasing scrutiny of the equine industry has highlighted concerns about such practices, contributing to discussions about the implementation of social licences to operate. Identifying strategies to mitigate stress and promote positive affective states is therefore increasingly relevant and important.

This study evaluated the effects of different auditory stimuli on horses by assessing both physiological and behavioural indicators, specifically, heart rate and micro-facial expressions.

Fifteen horses (8 mares, 7 geldings) from two locations were initially recruited, with twelve completing the study with full datasets. Baseline measures of heart rate (using a Polar H10 sensor) and facial expressions (coded via the EquiFACS system) were recorded at rest in stables and again in a solarium, where experimental trials took place. Over four consecutive days, horses were exposed to three auditory treatments and a silent control in a randomised order. The three auditory treatments were Mozart’s Fantasia in F minor (string performance, 432 Hz), pink noise, and equine-derived sounds (chewing and sneezing from a relaxed conspecific). The solarium was acoustically isolated to ensure all stimuli were novel. Each audio stimulus was played for 3 minutes and 10 seconds. Heart rate was recorded at 10-second intervals throughout, while facial expressions were captured on video and analysed using still frames extracted at matching intervals. Facial action units were scored in a binary manner. The initial 10 seconds of each recording were removed before analysis. 

Analysis of variance revealed a significant effect of treatment on heart rate. Post hoc Tukey testing indicated that heart rates were significantly lower during exposure to Mozart’s Fantasia compared to equine-derived sounds, with all but one horse showing this pattern. No significant differences were observed between the silent control and pink noise treatments, although two horses exhibited lower heart rates during pink noise relative to Mozart’s Fantasia.

Chi-squared analysis of facial expressions identified the muzzle region as most strongly associated with emotional state. Equine-derived sounds elicited increased frequencies of AD133, EAD101, AU5, and AD1, which are associated with vigilance and potential threat detection. In contrast, both Mozart’s Fantasia and pink noise treatments were associated with increased AU101, EAD101, and 001, alongside reduced AD1 and AU5, suggesting a more relaxed attentional state without directional focus. Notably, neither auditory treatment differed significantly from the silent control, potentially reflecting a pre-existing positive association with the solarium environment.

Overall, equine-derived auditory stimuli elicited more indicators of negative affective states, whereas Mozart’s Fantasia produced the most consistent reductions in heart rate and stress-related facial expressions. These findings suggest that classical music may have practical applications as an environmental enrichment tool to mitigate stress in managed horses.

Coding system based on EquiFACS: