Housing induced mood modulates reactions to emotional stimuli in sheep

The assessment of positive and negative short-term affective states (emotions) in animals and their modulation by long-term affective states (mood) is an on-going challenge. This study investigated the use of behavioural and physiological measures to assess emotions and their modulation by mood in sheep. To induce a more Positive and more Negative mood, two groups of sheep (n = 10 and 9) were kept under different housing conditions for 3 weeks. The Positive mood group was placed in a predictable environment rich in stimuli and with a calm human–animal interaction whereas sheep in the Negative mood group experienced a relatively barren and unpredictable environment with a harsher human–animal interaction. In the subsequent test week, 15 (8 and 7) experimental animals were exposed to four situations intended to elicit emotional reactions: separation from group members (negative valence), sham-grooming (low intensity negative), non-grooming (neutral), and grooming (positive valence). Data were tested for interactions between mood group and valence of the test situation using linear mixed models. More frequent ear-posture changes, a higher proportion of asymmetric ears, shorter inter-heartbeat intervals and higher respiration rates were observed during separation than during the other situations. Interactions between mood group and test situation showed that sheep in the Negative mood group reacted more negatively to the negative and more positively to the positive situation (proportion of asymmetric ears, inter-heartbeat interval, respiration rate). In conclusion, our findings suggest that sheep in a Positive mood reacted more weakly to an emotional situation independent of its valence, thus buffering negative events but also degrading positive events. In addition, this investigation showed the potential of using observations of ear postures and cardio-respiratory measures in sheep to assess the valence of short-term emotional states and their modulation by mood states.