Arianna De Marco at the University of Florence, Italy, and colleagues saw this behaviour among Tonkean macaques as they observed two captive groups over seven months.
When a macaque behaved aggressively â€“ by chasing, grabbing or biting, for instance â€“ De Marco chose a bystander at random and recorded whether it would “affiliate” with another macaque within 5Â minutes of the conflict ending. The primates were considered to be affiliating if they sat near, groomed or played with another bystander.
For comparison, De Marco observed the same macaque for 5Â minutes the next day at approximately the same time.
She found that bystanders were more likely to affiliate with another bystander after conflicts than at peaceful times. In one group, the macaques were almost three times as likely to do so, and in the other group almost seven times as likely.
After witnessing a conflict, bystander macaques tend to appear unusually agitated, scratching themselves more than normal, for instance. Once the macaques affiliated with each other, however, they seemed to calm down.
Previous research has found that after fighting, primates rarely attempt a reconciliation, which means that social groups of these animals need to either defuse violence in advance or find other ways to reduce tension. De Marco thinks that bystanders come together to prevent escalating aggression within the group.