Cooperating with friends helps monkeys relax

Stress levels of long-tailed macaques, a monkey species naturally occurring in South-East Asia, drop when they cooperate with a friend. Whereas it is known that friendship strengthens cooperative behaviour, this study by researchers from Utrecht and Vienna is one of the first that shows an intricate way in which cooperation regulates physiology accordingly. The results are published today in Royal Society Open Science. 

The team, led by Utrecht biologist Jorg Massen, investigated the link between cooperation, social relationships and stress in long-tailed macaques. The researchers conclude that cortisol levels, an indicator of stress, decrease when the monkeys cooperate with a befriended group member.

Cortisol levels

Fourteen long-tailed macaques, living in one big monkey group at the Biomedical Primate Research Centre in the Netherlands, were trained by lead author Martina Stocker to cooperate with other group members in order to get peanuts. Before and after the monkeys worked together, they voluntarily chewed on a saliva swab and as such provided saliva samples for the analysis of the hormone cortisol, a marker of stress

The study showed that after the monkeys had cooperated with friends their cortisol levels dropped. However, this was not the case when the friend was just present without performing a task together, nor when cooperating with a neutral individual. This rules out that cortisol levels decrease due to the mere presence of a befriended monkey, and highlights the importance of close social bonds.

Relaxing effect

Lead author Martina Stocker commented: “This relaxing effect of cooperative interactions with closely bonded individuals could be the underlying cause of the maintenance of cooperation as found in these macaques, but potentially also in other species and/or humans.”

Publication

Cooperation with closely bonded individuals reduces cortisol levels in long-tailed macaques
M. Stocker, M.-C. Loretto, E.H.M. Sterck*, T. Bugnyar, J.J.M. Massen*
Royal Society Open Science 7, 2020: 191056.

* researchers affiliated with Utrecht University