Macaques attend to scratching in others

Self-directed behaviours in primates as a response to increasing psychological or physiological stress are a well-studied phenomenon. There is some evidence that these behaviours can be contagious when observed by conspecifics, but the adaptive function of this process is unclear. The ability to perceive stress in others and respond to it could be an important part of sustaining cohesiveness in social primates, but spontaneously acquiring stress-related behaviours (and potentially emotional states) from all group mates via contagion could be maladaptive. To investigate this, a group of captive Barbary macaques, Macaca sylvanus, were presented with videos of conspecifics engaging in self-directed behaviour (scratching) and neutral behaviour. Behavioural responses as a result of exposure to the stimuli were compared (1) between familiar and unfamiliar individuals, and (2) within familiar individuals to consider the modulating effects of social relationships. Our results did not show contagious scratching in this species. However, there were differences in how individuals attended to the scratching stimuli. Subjects were more attentive to scratching videos than to neutral videos and familiar than unfamiliar individuals. Within the familiar individuals, subjects were more attentive to those to whom they were weakly bonded. We suggest that increased attention to scratching behaviours may be adaptive in order to monitor and avoid stressed group mates, whose subsequent behaviour may be unpredictable and aggressive. Monitoring group mates who are not allies may also be adaptive as they may pose the biggest risk. These findings will help increase our understanding of subtle cues that can be communicative in primates, and also the evolutionary steps towards understanding others.