To date, most studies on animal emotions have focused on the assessment of negative emotional states, and there is a lack of approaches to characterising positive emotional states. The aim of this investigation was to measure differences in ear and tail postures in sheep exposed to situations likely to induce states of negative, intermediate and positive emotional valence.
Nineteen female sheep were observed in emotion-eliciting situations in two experiments. In the home-pen experiment, ear and tail postures were observed during separation from group members (negative situation), during rumination (intermediate), and while feeding on fresh hay (positive situation). In the fodder experiment, individual sheep were conditioned to anticipate the delivery of standard feed. Once familiar with this experimental condition, they were offered either the standard feed (control treatment), unpalatable wooden pellets (negative treatment), or energetically enriched feed mixed with preferred feed items (positive treatment). Ear and tail postures of sheep were recorded during the final 6min preceding feed delivery (anticipation phase) and for 6min during feed delivery (feeding phase). Data were analysed using linear mixed-effect models.
In the home-pen experiment, sheep separated from group members showed a high number of ear-posture changes and a high proportion of forward ears compared to hay feeding, during which ears were mainly passive. In the fodder experiment, the total number of ear-posture changes was generally high during the anticipation phases, slightly lower during delivery of the wooden pellets, and clearly reduced during the delivery of standard and enriched feed. A higher proportion of passive ear postures occurred when standard feed and enriched feed were offered compared to the delivery of wooden pellets. The proportion of asymmetric and axial ear postures was influenced by the sequence of testing of the different feeding treatments, with a higher proportion of asymmetric and a lower proportion of axial ear postures during the first exposure to either the wooden pellets or the enriched feed. A high proportion of the sheep's tails being raised was only observed during separation from group members.
In both experiments, frequent ear-posture changes were most clearly associated with situations inducing negative states, and a high proportion of passive ear postures with situations likely to induce positive emotional states. Unfamiliarity influenced emotional reactions towards a more negative appraisal. A raised tail only appears to occur in specific situations, and was not useful for distinguishing emotional valence. Apart from the need for further validation, observations of ear-posture changes seem to be a promising approach for assessing emotional reactions in sheep.