Chronic stress induces pessimistic-like judgment and learning deficits in sheep

Chronic stress can be described as a long-term negative affective state induced by an accumulation of negative emotional experiences that alters an individual's interactions with the environment. In humans, chronic stress induces both persistent judgment biases and learning deficits. We investigated whether similar cognitive biases could occur in sheep. Twenty-four lambs were subjected to a chronic stress treatment for 9 weeks (exposure to various unpredictable, uncontrollable and aversive events regularly encountered in ordinary agricultural practices) and 24 other lambs were used as a control group. Before treatment all the lambs had been trained to approach or avoid a food bucket depending on its location. After treatment, the lambs were individually exposed to two tests: (i) a food bucket approach/avoidance test, by testing the response of lambs to ambiguous locations of the bucket, i.e. a judgment bias test, (ii) a learning test consisting of training on six consecutive days to distinguish between two visual stimuli differing in their color and shape (acquisition phase) and re-exposure to the test conditions 2 days after the end of the acquisition phase (recall phase). In the judgment bias test, treated lambs took longer to approach the ambiguous locations of the bucket. During the learning test, treated lambs had lower numbers of correct choices than control lambs in the acquisition and recall phase. Hence sheep experiencing repeated aversive events demonstrated pessimistic-like judgment biases and learning deficits. These may be useful measures to assess long-term emotional states in animals.