Observing Aggression Increases Aggression In Semi-Free Ranging Barbary Macaques

In many social living species, seeing conspecifics interacting can alter the behaviour of bystanders, leading to social contagion – the spread of behaviour or emotion among group members. Among primates, studies of a small number of species have explored bystanders’ responses to observing aggressive interactions, finding evidence that individuals that see such interactions are more likely to subsequently engage in aggressive behaviours themselves. To increase the taxonomic breadth of this body of research, working with semi-free ranging adult female Barbary macaques ( Macaca sylvanus ) at the Trentham Monkey Forest, Stoke-on-Trent, UK, we assessed bystanders’ responses to observing naturally occurring aggressive interactions. Data were collected under two conditions: (i) after observing an aggressive interaction between conspecifics and (ii) during a matched-control period, before which individuals did not observe aggression. Bystanders were significantly quicker to initiate an aggressive interaction themselves after observing an aggressive interaction than if they had not, providing evidence of behavioural contagion. There was no effect of observing aggression on the rates of self-directed behaviour (an indicator of anxiety), suggesting the negative emotional states associated with involvement in aggressive interactions did not spread to bystanders. The results of this study provide new insight into the nature and importance of visual contagion of behaviour among primates.