Poker Face: Discrepancies in behaviour and affective states in horses during stressful handling procedures

Correct assessment of stress in horses is important for both horse welfare and handler safety during necessary aversive procedures. Handlers depend on behaviour when judging how well an individual is tolerating stressful procedures such as loading or veterinary intervention. However, evidence suggests that behaviour may not accurately reflect affective states in horses. This may be explained by individual differences in coping styles, which have tentatively been identified in horses. The current study assessed whether behaviour during two novel handling procedures was associated with physiological indicators of stress. Core temperature, discrepancy in eye temperature and heart rate variability (HRV) were compared with compliance and proactivity shown by horses during two novel handling tests (n = 46). Test A required subjects to cross a large blue tarpaulin on the ground. Test B required subjects to walk through plastic streamers suspended overhead. Physiological indicators of stress did not correlate with time taken to complete the handling tests. This indicates some subjects crossed an object they found aversive. Crossing time may be influenced more by stimulus-control than the level of aversion experienced. The level of proactivity shown was not associated with HRV, HR, core temperature or the discrepancy in temperature between eyes. This suggests that proactive horses, which appear more stressed, show similar stress responses to more reactive individuals. These findings support previous research indicating that behaviour commonly used within the equestrian industry may not provide reliable indicators of a horse's ability to tolerate a stressful procedure. The influence of training and the extent to which a horse is under stimulus-control may over-shadow inherent emotional responses, with implications for handler safety and horse welfare.