Looking stressed can help keep the peace

Scratching is more than an itch — when it is sparked by stress, it appears to reduce aggression from others and lessen the chance of conflict.

Scratching can be a sign of stress in many primates, including humans.

Research by Jamie Whitehouse from the University of Portsmouth, is the first to suggest that these stress behaviours can be responded to by others, and that they might have evolved as a communication tool to help social cohesion.

The research, published in Scientific Reports, raises the question whether human scratching and similar self-directed stress behaviours serve a similar function.

Jamie said: “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.”

The research team conducted behavioural observations of 45 rhesus macaques from a group of 200, on the 35-acre island of Cayo Santiago, Puerto Rico. The team monitored the naturally occurring social interactions between these animals over a period of eight months.

The researchers found that scratching in the monkeys was more likely to occur in times of heightened stress, such as being close to high-ranking individuals or to non-friends.

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.

Jamie said: “As scratching can be a sign of social stress, potential attackers might be avoiding attacking obviously stressed individuals because such individuals could behave unpredictably or be weakened by their stress, meaning an attack could be either risky or unnecessary.

“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.”

The researchers expect the findings will lead to a better understanding of stress and the evolution of stress in humans as well as how we manage stress in captive animals.

Stress behaviours buffer macaques from aggression

Jamie Whitehouse, Jérôme Micheletta, Bridget M. WallerDepartment of Psychology, Centre for Comparative and Evolutionary Psychology, University ofPortsmouth, U.K.Corresponding author: Jamie Whitehouse, Department of Psychology, University of Portsmouth,King Henry Building, King Henry I Street, Portsmouth PO12DY, United Kingdom.Email: jamie.whitehouse@port.ac.uk

Abstract

Primates (including humans) scratch when stressed. So far, such scratching has beenseen as a by-product of physiological processes associated with stress, and attributedproximate, regulatory function. However, it is possible that others could also use thisrelationship between scratching and stress as an indication of the animals stress state,and thus potentially have additional social function. As a test of this theory, wemeasured the production of, and social responses to scratching in a group of free16ranging rhesus macaques (Macaca mulatta). Firstly, we found that the occurrence ofscratching was more likely around periods of heightened social stress, such as beingin close proximity to high-ranking individuals, or non-friends. Secondly, whenmacaques scratched, subsequent interactions were less likely to be aggressive andmore likely to be affiliative. Potential attackers may avoid attacking stressedindividuals as stressed individuals could behave unpredictably or be weakened bytheir state of stress (rendering aggression risky and/or unnecessary). Observable stressbehaviour could therefore have evolved, or be evolving, with social pressures whichprovide adaptive value by reducing the potential for escalated aggression, benefitingboth senders and receivers by facilitating social cohesion. This basic ability torecognise stress in others could also be an important component in the evolution ofsocial cognition such as empathy.

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