Judgement bias: A cognitive measure of affective state in sheep

Abstract

Sheep welfare is predominately measured by identifying behavioural and physiological changes. While informative, they can often be difficult to interpret, identifying the arousal of a sheep in a particular situation but not the valence (positivity or negativity) of the affective state being experienced. Judgement bias has been proposed as a cognitive measure of affective state valence in animals. The aim of this thesis was to adapt a method of assessing judgement bias for use in sheep to identify cognitive changes influenced by affective states. In all studies, sheep were trained to a go/no-go operant task where they learnt to approach (go response) a bucket when placed in one corner of the testing facility to receive a positive reinforcer (feed reward), and not approach it when in the alternate corner (no-go response) to avoid a negative reinforcer (exposure to a dog or fan forced blower). Judgement bias testing involved exposing the sheep to ambiguous bucket locations between the two learnt ones. The sheep had to judge which way to respond to these ambiguous locations (go/no-go), which would give insight into their affective states. In the first study a restraint and isolation stressor (RIS) for 6 h/day on 3 consecutive days generated a significantly more positive judgement in sheep compared to controls. Two further studies, using a chronic, intermittent treatment and a pharmacological treatment using a serotonin antagonist respectively, generated more pessimistic-like judgement biases in the sheep compared to that of the controls. These studies suggest that judgement bias can identify affective state changes in sheep and that the response differs depending on the nature of the stress. Furthermore, the unexpected result generated from the RIS treatment suggests that judgement bias can be used to measure differently valenced affective states in sheep. Another study conducted into the method itself showed that the sheep learn not to approach the unreinforced ambiguous locations when repeatedly exposed to them and this affects their judgement of the bucket locations. This means that there are a limited number of times that the sheep can be exposed to the testing situations. Despite this and other methodological concerns discussed, these results strongly suggest that judgement bias can be used as a measure of affective state in sheep. The use of this method could help to improve the welfare assessment of sheep in the future, especially if used in conjunction with other measures of affective state.