The mane factor: Compliance is associated with increased hair cortisol in the horse

Abstract

Equine personality tests enable the selection of horses for roles based on their habitual behavioural responses to challenges, aiming to maximise performance and safeguard equine welfare. However, existing research has identified that behavioural responses to acute challenges do not correlate well with stress physiology in the horse. The aim of this study was to investigate whether chronic stress physiology is related to habitual compliance in horses. Hair cortisol concentration (HCC), a long-term biomarker of hypothalamic–pituitary–adrenal axis activity and stress, was assayed in mane hair from the withers, midpoint of the neck and poll of 24 riding school horses (66% male, mean age 13 ± 4 years). Three caregivers provided scores for Agreeableness, Neuroticism, Extroversion, Gregariousness Towards People and Gregariousness Towards Horses for all subjects using a validated equine personality questionnaire. A gamma GLMM was used to model HCC as a function of personality scores and potential confounders of HCC (age, sex, hair colour and sampling location). Individual horses were fitted as random intercepts in the model. Manual backward selection was used to identify the best-fitting plausible model. Mean HCC was 3.8 ± 1.2 pg/mg. The final model only retained Agreeableness and sampling location as explanatory variables of HCC. A positive association was found between Agreeableness (reflecting habitual compliance) and hair cortisol concentration (t = 2.7, p = 0.01). Basal cortisol levels are known to drive sensitivity to punishment in humans. Therefore, horses with higher basal cortisol may be more sensitive to the aversive cues associated with negative reinforcement and consequently be more compliant. However, it is also possible that habitually compliant behaviour leads to higher chronic stress levels through increased exposure to stressors, as more subtle expressions of discomfort are not recognised by handlers. This result has important implications for welfare in the context of horse-human interaction and warrants further investigation to clarify whether a causal link exists between habitual compliance and higher levels of chronic stress. There was also a weaker but significant impact of sampling location (t = 2.1, p = 0.03; higher HCC at the poll), highlighting the importance of carefully standardizing sampling location for HCC analysis.