Grooming, social rank and ‘optimism’ in tufted capuchin monkeys: a study of judgement bias

• We used a judgement bias test to study emotional responses in capuchin monkeys.
• Receiving grooming had no immediate detectable effect on ‘optimism’.
• Monkeys that received overall more grooming were more ‘optimistic’.
• High-ranking monkeys were also more ‘optimistic’.

Studying the emotional consequences of social behaviour in nonverbal animals require methods to access their emotional state. One such method is provided by cognitive bias tests. We applied a judgement bias test to tufted capuchin monkeys, Sapajus sp., to evaluate (1) whether receiving grooming was associated with a short-term increase in ‘optimism’ (that is, a positive bias in the interpretation of ambiguous stimuli) and (2) whether interindividual differences in ‘optimism’ were related to dominance rank or overall rates of social and nonsocial behaviour. Receiving grooming had no detectable immediate consequences, but increased ‘optimism’ was observed in dominant monkeys and in monkeys that received overall larger quantities of grooming. These results provide only partial support for the hypothesis that a system of emotional bookkeeping underlies the capacity of group-living animals to reciprocate cooperative interactions.