Scratching can be a sign of stress in many primates, including humans, and may may have evolved as a communication tool to help social cohesion in monkeys, a study has found.
The findings showed that scratching in the monkeys is more than an itch and was more likely to occur in times of heightened stress, such as being close to high-ranking individuals or to non-friends.
During such stressful experiences scratching appeared to reduce aggression from others and lessen the chance of conflict, suggesting that it might have evolved as a communication tool to help social cohesion.
“Observable stress behaviours could have evolved as a way of reducing aggression in socially complex species of primates. Showing others you are stressed could benefit both the scratcher and those watching, because both parties can then avoid conflict,” said Jamie Whitehouse from Britain’s University of Portsmouth.
The research also raises the question whether human scratching and similar self-directed stress behaviours serve a similar function.
Further, stress scratching significantly lowered the likelihood of a scratching monkey being attacked.
The likelihood of aggression when a high ranking monkey approached a lower ranking monkey was 75 per cent if no scratching took place, and only 50 per cent when the lower ranking monkey scratched.
Scratching also reduced the chance of aggression between individuals who did not have a strong social bond.
“By revealing stress to others, we are helping them predict what we might do, so the situation becomes more transparent. Transparency ultimately reduces the need for conflict, which benefits everyone and promotes a more socially cohesive group,” Jamie added.
For the study, published in Scientific Reports, the team conducted behavioural observations of 45 rhesus macaques from a group of 200. The team monitored the naturally occurring social interactions between these animals over a period of eight months.