Physiological expression of emotional reactions in sheep

With the aim of judging emotional valence from an animal's perspective, multiple physiological variables were recorded in sheep when they were exposed to situations likely to induce negative and positive emotional states. Fourteen sheep were conditioned for several weeks to anticipate the delivery of standard feed. In three experimental trials carried out thereafter, the animals' expectations regarding feed quality were either fulfilled by offering the familiar standard feed (control), frustrated by giving unpalatable wooden pellets (negative treatment) or surpassed by delivering enriched feed (positive treatment). Heart rate, root mean square successive difference (RMSSD), respiration rate, body-surface humidity, body-surface temperature and percentage of eye white were recorded prior to the delivery of feed (anticipation phase) and during the delivery (feeding phase) of either standard feed, wooden pellets or enriched feed. Data were analysed using linear mixed-effects models. Heart rate, respiration rate and variability of body-surface humidity were high during the delivery of wooden pellets and low during the feeding phases with standard and enriched feed; RMSSD showed an inverse pattern. In addition, heart rate was increased during the first feeding phase after the one with standard feed, independent of its presumed valence, whereas differential reactions were observed in the second feeding phase after standard feed. The results show that presumed negative and positive emotional states in sheep differ in their physiological reactions. Despite a need for validation in additional situations, the combination of heart rate, respiration rate, RMSSD and body-surface humidity appeared to be most useful for assessing physiological correlates of negative and positive emotional reactions in sheep.