Can we detect stress-induced cognitive bias in farm animals? Development of a method for sheep

Abstract

An individual's cognitive evaluation may be altered according to its mental state. By providing an ambiguous situation for an animal to assess, insight into their mental state may be able to be obtained. In order to test for this effect in sheep, twenty ewes were trained to learn that a feed bucket placed in one pen corner resulted in a positive reward on approach; however, when the bucket was in the alternate corner, a negative reinforcer (sight of a dog) was presented. Once reaching the learning criterion, sheep were assigned to treatment or control groups. Treatment involved a restraint and isolation stressor (RIS) for 6h/d on three consecutive days. Putative stress-induced cognitive bias was tested by providing ambiguous "probe" buckets in three positions between the positive and negative buckets. Sheep approach behaviour to the five bucket positions was recorded. Animals were tested before RIS, following daily RIS, and the day afterwards. Plasma cortisol and behavioural responses in a novel arena were also measured. A graduated response in approach behaviour to the buckets indicated that the testing scenario successfully made the animals "assess" the probe situations.