How monkeys make friends and influence each other

The best of friends. Hung Chung Chih/Shutterstock

For us humans, getting involved in an aggressive conflict can be costly, not only because of the risk of injury and stress, but also because it can damage precious social relationships between friends – and the same goes for monkeys and apes.

Just like humans, they also form selective long-term, reciprocal bonds that share many parallels with human friendship. And in the primate world, aggression can also be detrimental to these relationships because it decreases tolerance and the rate of friendly interactions.

Like human families, for monkeys and apes, the day-to-day business of living in a group inevitably brings quarrels. Disputes could be over who gets the shady spot to rest in, who’s in charge, who to groom, who to mate with, who to huddle up with in the cold or where to feed. Life can be harsh. But luckily, primates have a whole arsenal of strategies up their sleeves to forestall, or mitigate the costs of, aggression.

These range from formal submission and calming tensions before they escalate, to mediation and policing interventions during a conflict. But if aggression is unavoidable and a fight occurs, opponents have another option. Because just like humans, primates can also repair their relationship through reconciliation – to help reestablish friendly contact.

Conflict resolution

Reconciliatory behaviour was first recognised by Frans de Waal in the 1970’s in a seminal study of post-conflict behaviour in chimpanzees. On the surface, reconciliation boils down to friendly contact between opponents soon after a fight but it also seems to do more than just end the conflict.

Studies have found that reconciliation has emotional consequences in primates, reducing indicators of distress and anxiety – such as elevated heart rate and scratching – back to baseline levels.

‘Hi friend’. Nick Fox/Shutterstock

Post-conflict anxiety levels in primates have also been found to be related to the quality of the relationship between the former opponents. Among humans, this makes perfect sense, if you argue with a close friend, you’re much more stressed than if you’ve argued with a passing acquaintance.

Researchers have also found that reconciliation reduces the likelihood of renewed aggression. But perhaps most importantly, reconciliation appears to restore tolerance and cooperation between friends.

Making amends

Friendships improve health and increase survival and reproductive success in many species, such as dolphins, horses, birds and primates, so it’s not surprising that mechanisms have evolved to alleviate damage to a relationship caused by aggression.

The fact that reconciliation is common to many social species shows how deep-rooted our own tendency is for peace-making. But it would seem that some techniques are actually learned rather than innately acquired.

In an innovative experiment some years ago, de Waal showed that the reconciliatory behaviour of quarrelsome, juvenile rhesus macaques, could be increased threefold after a few months of co-housing with more easygoing, peacemaking stumptail macaques.

‘Give us a kiss’. olga_gl/Shutterstock

So conciliatory tendency seems to be a social skill that is acquired through juvenile experience, rather than an innate behaviour. And a study just published in the International Journal of Primatology by one of my PhD students, supports this view.

Indeed, adult chimpanzees often reconcile using grooming and specific reconciliatory behaviours that are not seen in other contexts – such as the mouth-to-mouth kiss. So if reconciliation was an innate behaviour, we’d expect to find that juvenile chimpanzees mimicked that of the adults – but in our research we did not find this to be the case. Instead, after a fracas, young chimpanzees reconciled the best way they knew how – through play. They also lacked the reconciliatory finesse of their elders.

And while it’s well known that adult chimpanzees are more likely to reconcile with their more valuable friends than non-friends, the young chimpanzees didn’t seem to make this discrimination yet, suggesting they’ve still got a lot to learn.

Monkeying around

I remember observing Japanese macaques as a student, under a 35°C sun, high humidity and oddly enough sweaty knees (who knew knees could sweat). I watched one young female monkey trundle up a slope and jump out from between some bushes, straight into the lap of sleepy Kusha. Surprised, Kusha threatened and lunged at Ai who cowered submissively before running off to a safe distance about four metres away.

Friends who bathe together stay together. norikko/Shutterstock

I looked on as, after only a few seconds, Kusha walked up to Ai sat down next to her and started to groom her for half a minute. Relaxed, Ai laid down and let Kusha groom her side. Then they swapped over and Kusha laid down while Ai groomed her leg. Throughout the whole encounter they exchanged friendly lip-smacks – these are rapid opening and closing of the lips – signalling their friendly intentions to each other.

When it comes to us humans, while we probably don’t fancy grooming every person we’ve fallen out with – or kissing them – it’s clear that when it comes to arguments peacemaking is the preferred option for both species. So next time you fall out with your co-worker maybe try as the primates do and work on that conflict resolution – a bit of lip smacking, a bit of a back scratch and hopefully you’ll be friends again in no time.