Pigtail macaques live in communities often near the brink of violence. Scientists have shown how targeted aggressive behaviour by just a couple of monkeys can descend into an all-out brawl.

Close observation of a group of 48 pigtail macaques in captivity has shown that their social group is permanently close to the brink of violence, a new study finds. It only takes aggravating behaviour by one or two rebel monkeys to make the group boil over into fighting.

Keeping the balance

Many groups of animals sit close to a "critical point", which makes them more sensitive and responsive to sudden environmental changes. This could confer an evolutionary advantage on them, leading to quick group reflexes to take action when in danger, for example.

But sometimes actions that push a group past its critical point don't give them an evolutionary advantage, the authors write in the paper. The balance between sensitivity to danger and volatility in a group is a delicate one, and the two can be traded off if the distance of the group from disaster can be controlled within the group.

Researchers at the Santa Fe institute watched the monkeys over a four-month period. For each fight that broke out, they monitored which macaques took part and which didn't, and who started it. They also used statistical techniques to see how close the group was to critical point.

Some skirmishes were kept between two or three macaques. For a really big fight to start, involving as many as 30 adults, about four or five individuals to get agitated. Their aggression would then quickly spread and lead to a mass brawl.

Monkey police

Some macaques were in the habit of cooling things down when it looked like a fight might break out. They helped to reduce the temperature of the group by picking up on aggressive behaviour and stopping it before it had a chance to spread. There were four of these "police" macaques in the group, three male and one female. This could be a way for the monkeys to control how close they are to the critical point for mass fighting.

"All biological systems need to balance stability and robustness with the need for rapid adaptive change," the authors write in the paper published in the journal Nature Communications.

"Tuning [the group's distance from critical point] allows for switching between stability and criticality, providing a means for accessing alternative social structures that may be more appropriate if and when the environment should change."