Are hungry sheep more pessimistic? The effects of food restriction on cognitive bias and the involvement of ghrelin in its regulation.

Food restriction is considered to be a welfare issue in extensively reared animals. However, the effects of food restriction on the affective state, and its physiological regulation, are unknown. In Experiment 1, we aimed to assess the effects of increased plasma concentrations of acyl-ghrelin on judgement bias (an indicator of affective states) by fasting sheep for 24h or by ghrelin administration. In Experiment 2, we aimed to assess the effects of chronic food restriction on judgement bias and attention bias towards a food-related cue. For the judgement bias test, sheep were trained in an arena to approach a positive location cue associated with conspecifics and not approach a negative location cue associated with a dog. Three non-trained, non-reinforced ambiguous location cues were situated between the positive and negative locations. Attention bias towards a food-related cue was assessed by placing an empty food bucket against the wall of the arena halfway between the entry point and the positive location. In Experiment 1, sheep were divided into three treatments; 24h fast, ghrelin administration or control. Judgement bias, locomotor activity and plasma cortisol concentrations were assessed. The ghrelin treated group tended to express a more pessimistic bias compared to the control group (P<0.1), and plasma cortisol concentrations tended to be increased (P<0.1). In Experiment 2, sheep were subjected to a high feeding level (HF) or low feeding level (LF) for 7days. The LF group tended to show a more optimistic judgement bias (P<0.1). When the food-related cue was presented, LF ewes took longer to reach the positive location (P<0.001), spent longer with their head inside the bucket (P<0.001) and more time interacting with the bucket (P<0.01). This study provides preliminary evidence that food restriction alters judgement bias and attention bias towards a food-related cue which may indicate altered affective states of sheep.