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The role of behavioural assessment in determining 'positive' affective states in animals.

Abstract
 The assessment and promotion of positive affective states as opposed to consideration of negative states alone has now become an accepted component of animal welfare thinking. However, methods for assessment of positive affective state are comparatively underdeveloped compared with their negative counterparts. Due to the current lack of established physiological methods for assessing positive effect in animals, behavioural measures are likely to be the closest proxies. In this paper, we broadly review spontaneous behavioural measures that have been utilized in mammals to determine positive affect. This includes luxury behaviours, facial expression, vocalizations and laterality. Play and anticipatory behaviours expressed in animals have received greatest research attention. However, these methods require care in interpretation due to the influence of age and other factors, such as social status on their performance. These methods may also be subjective if descriptions of individual behaviours are not well defined and validated. Methods that are amenable to automation may reduce subjectivity. Automation will also aid in more widespread implementation as an on-enterprise welfare assessment tool. There is a pressing need to develop simple and objective indices of positive affect. The use of vocalizations and facial expressions may fulfil these criteria. Future research attention should be directed towards further characterising these methods.