I feel your pain: individual differences in welfare indicators after castration in horses

Personality affects both experience and expression of pain and the welfare impact of castration on horses is poorly understood. Therefore, the current study observed 19 horses to determine: the welfare impact of standard castration on horses; whether individuals consistently vary in their behavioural and emotional responses to pain; the influence of personality on behavioural and physiological responses to pain; whether Horse Grimace Scale (HGS) indicates how individuals feel about painful experiences. Eye temperature (IRT), salivary cortisol, HGS and a pain ethogram were measured at intervals before, throughout and during recovery from castration. IRT (p < 0.005), Cortisol (p < 0.024), HGS (p < 0.03) and Maintenance behaviour (p < 0.004) significant changed from baseline. Physiological and behavioural responses to castration were varied but not consistent within individuals. Veterinarian influenced responses, presumably reflecting the importance of clinician's skill. Personality explained differences in cortisol responses with Neuroticism negatively (estimate=-0.275; p = 0.035), and Extroversion positively (estimate=0.406; p = 0.001) associated with the magnitude of response to castration. HGS was not confounded by personality suggesting that this pain indicator may be resilient to individual differences in pain expression and appears to reflect underlying affective pain states as it was associated with cortisol (r = 0.568, p = 0.027). Therefore, it is potentially an important tool in recognition of pain at an individual level. Further research should be done utilising a larger sample with greater standardisation of castration method to determine both the effect of baseline welfare on pain resilience and the sensitivity of Grimace Scales as an indicator of suffering during painful experiences.