In Kibale National Park, Uganda, female chimps have taken to carrying sticks around with them. There’s nothing obviously unusual about that – chimps are clever tool-users, who use sticks as probes, projectiles and spears. But these chimps aren’t doing very much with their sticks – they simply hold and cradle them while they go about their usual business.

It might seem like a far-fetched idea, but the duo make their case strongly. For a start, the sticks have no other obvious use. Kahlenberg and Wrangham spent 14 years watching the chimps of Kibale’s Kanyawara community. In that time, they’ve seen the apes using sticks in all sorts of ways but around 40% of the time, they just carry sticks (or