Monkeys increase scratching when encountering unexpected good fortune.

Elucidating how ancestral behavioural traits have been repurposed for psychological and social functions is critical to advancing our understanding of human behavioural evolution. Self-scratching, originally a hygienic response and known to exhibit social contagion, serves as a model for this process. Although human scratching behaviour is traditionally linked to negative emotions, evidence from non-human animals has produced inconsistent results, casting doubt on its association with negative emotions. Here, we examined scratching in Japanese macaques (Macaca fuscata) during cognitive tasks. In Experiment 1, we manipulated reward outcomes by introducing rare, unexpected increases ("fortunate" condition) or omissions ("unfortunate" condition) of rewards; scratching increased significantly under the fortunate condition, but not the unfortunate one. In Experiment 2, we presented unexpected visual stimuli without altering reward outcomes and observed increased scratching. Our results challenge the traditional view that scratching is primarily linked to negative emotions. Our findings suggest that scratching, which originally evolved as a response to unexpected skin stimulation, may later have been repurposed as a coping mechanism to manage heightened uncertainty.