Behavioural and physiological assessment of positive and negative emotion in sheep

The assessment of emotional states in animals, particularly positive ones, remains a scientific challenge. We investigated differences in behavioural and physiological measures recorded in sheep, Ovis orientalis aries, during situations likely to coincide with negative, intermediate and positive emotional valence. Reactions of 15 sheep were observed during separation from group members (negative valence), standing in the feeding area (intermediate valence) and being voluntarily groomed by a familiar human (positive valence). Several ear postures, relative eye aperture, cardiorespiration and body surface humidity and temperature were recorded continuously for up to 4 min in each experimental situation. Data were analysed using linear mixed-effects models, and correlations were calculated between ethophysiological measurements. Groomed sheep showed few ear posture changes, low proportions of forward ear postures, low relative eye aperture and a low variance in body surface humidity. The values of most of these measures increased linearly towards standing in the feeding area and further towards separation from group members. Conversely, groomed sheep showed high proportions of axial ear postures, long mean interheartbeat intervals and high heart rate variability, with values declining linearly towards the negative situation. In addition, behavioural and cardiac measurements correlated moderately to strongly. In conclusion, emotional valence in sheep could be differentiated by both behavioural and physiological measures. Based on our data, it may be possible to replace some physiological measures with observations of ear postures. The findings provide valuable insights for assessing both negative and positive emotion in animals, which can help to promote positive experiences in captive housing conditions.