Repeated exposure to positive events induces optimistic-like judgment and enhances fearfulness in chronically stressed sheep

Chronic stress in farm animals can induce detrimental effects in emotional sensitivity and cognitive function, for example, by altering fearfulness and eliciting pessimistic-like judgment. Based on a few experiments carried out on laboratory animals, we suggest that exposing chronically stressed sheep to an enriched environment could alleviate the detrimental stress-induced effects on fearfulness and judgment. Fifteen lambs (Chronically Stressed lambs, CS) were submitted for 7 weeks to a treatment known to induce chronic stress (i.e. repeated and unpredictable exposures to various uncontrollable and aversive events every day and/or night). Fifteen other lambs (lambs exposed to Negative and Positive events, NP) were also submitted to the same stressful treatment and, for the last 4 weeks of the stressful treatment, exposed daily to various positive events, i.e. wool brush, positive contact with humans and anticipation of food by a light announcing daily food distribution. Before the treatment, all lambs were individually trained to spatially differentiate between positive and negative reinforced bucket locations with a go/no-go approach response. At the end of the treatment, each lamb was exposed to a judgment test using the same go/nogo design by offering it three ambiguous and unreinforced bucket locations in addition to the two learnt locations. In addition, lambs were exposed to two standard tests of fearfulness (exposure to novelty and suddenness) and human approach test. NP lambs displayed a positive judgement bias: they approached the bucket placed at the ambiguous location that was closest to the positive one faster than CS lambs (3.0 s± 0.3 vs. 5.9 s± 1.6; P = 0.006). NP lambs approached the human more than their counterparts (15.0 ± 1.4 vs. 7.7 ± 1.5 times in human zone during 10 min; P = 0.003) and were also more reactive to novelty and suddenness. Thus, repeated exposure lambs to positive events during a stressful period induces an optimistic-like judgment but also increases emotional reactivity. Further studies are thus necessary before promoting such a positive treatment as a method to alleviate negative effects of chronic stress