Oxytocin levels rise in groups of chimpanzees when conflict with other groups might be near Steve Bloom Images/Alamy Stock Photo

Is the fabled “cuddle hormone” really a “warmone”? Oxytocin levels surge in troops of chimpanzees preparing for conflict with rival groups to defend or expand their territory.

The finding is at odds with the prevailing image of oxytocin as something that helps strengthen bonds between parent and infant, or foster friendships. But given its capacity to strengthen loyalty, oxytocin could also be a warmonger hormone that helps chimps galvanise and cooperate against a common enemy.

Catherine Crockford of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and her colleagues monitored two rival groups of chimpanzees in the Taï National Park in Ivory Coast, each containing five males and five females, for prolonged periods between October 2013 and May 2015.


Thanks to trust built up between the team and the chimps, the team could safely track and video the groups – even during conflict, observing at close quarters what was happening. Crucially, the team was also able to pipette up fresh samples from soil when chimps urinated.

Border checks

The samples revealed that oxytocin levels surge in the mammals whenever the chimps on either side prepared for confrontation, or when either group took the risk of venturing near or into rival-held territories. These surges dwarfed the oxytocin levels seen during activities such as grooming, collaborative hunting for monkey prey or food sharing.

“We found that oxytocin levels in samples related to territorial activity are extremely high,” says Crockford. “They approached double the levels seen in the ‘control’ activities, both for border control and actual conflict.”

The findings echo the results of lab research in which oxytocin levels were raised when teams of human volunteers were pitted against each other in competitive games.

But the chimpanzee research is the first to demonstrate that oxytocin rises during real conflict, and in a species other than humans.

“It confirms in another species the findings we got in our lab with humans,” says Carsten de Dreu of Leiden University in the Netherlands. “We found when we gave oxytocin to men in an intergroup setting, they became more affiliative to their own group members and more protective against outsiders.”

Defend against threats

Crockford and her colleagues suggest that the socially galvanising effect of the hormone has evolved from its more primitive role in stimulating mothers to defend their offspring.

“There’s logic behind this when you consider behaviours that help mothers form a bond of trust with their offspring, then physically protect them from threats,” says Crockford. And the oxytocin surges were as strong in females as in males. “It’s not a male-only phenomenon,” she says.

Crockford suspects that oxytocin may prime the social parts of the brain and reduce anxiety to facilitate cooperation. But what triggers its production remains unknown. She thinks it might be testosterone, a hormone that can support aggression against outsiders. “In our next initiative, we’re going to measure both to see if there’s a link,” she says.

De Dreu says that even if oxytocin does turn out to incite confrontational behaviour in humans, it would be difficult to neutralise it without losing the positive benefits. “There are no simple solutions to war and conflict, and if you dampened the response to ‘outsiders’, you would also dampen the affiliative benefits,” he says. “So be careful what you wish for.”

Journal reference: PNAS, DOI: 10.1073/pnas.1616812114

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