During an eight-month-long research trip to the Democratic Republic of Congo (DRC) and Zambia, we studied consolation: the way bonobos and chimpanzees comfort each other after conflicts and distress. But early on, we kept noticing something that didn’t quite fit.

The animals were exchanging a lot of friendly physical contact, embracing, touching and kissing, in competitive situations before any clear conflict or distress signals had broken out. We started wondering what all that touching was actually for.

Our new research into how great apes manage social stress suggests it could be quite important, and that this behaviour dates back at least 6 million years, long before our species evolved. Like us, our closest ape cousins, the bonobos and chimpanzees, also navigate stressful and competitive situations every day.

One example is when they discover a tree laden with ripe fruit. Everyone wants to eat and conflicts of interest can often lead to fights. But something else caught our attention. More often, apes reach out to one another with friendly touch: embracing, patting and making gentle contact.

Our colleague Edwin van Leeuwen, a Dutch biologist, had developed a feeding task to measure social tolerance across great ape groups. This task is called the peanut swing. It involves filling a bamboo trough with peanuts and swinging it over the fence so the food lands in front of the apes. This spreads peanuts evenly across a small area while researchers record who gets food, and how much. We used this test to measure whether groups could share resources peacefully in close proximity.

Because the apes watched the trough being filled,........

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