Chimps have spontaneously figured out how to use a stick to mash a potato. The finding could prompt a rethink of how tool use develops in primate societies.
Wild chimpanzees in Guinea climb palm trees to eat the trees’ “hearts”, which look like white asparagus. They use sticks to mash the hearts before eating them, like cooks using pestles and mortars. Chimps elsewhere don’t do this, suggesting that the behaviour is cultural and that Guinea chimps pick it up by copying each other.
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However, Claudio Tennie of the University of Tübingen in Germany is sceptical about copying. Many studies have demonstrated copying among chimps by showing them how to get food out of a puzzle box and seeing if they can then do it. Tennie says these studies are flawed because the chimps may simply be grasping how such boxes work and then figuring out how to open them themselves.
To see whether chimps can work out such tricks on their own, Tennie and his colleague Elisa Bandini studied semi-wild chimps at the Chimfunshi Wildlife Orphanage in Zambia. The chimps had never seen stick-pounding. Bandini and Tennie gave them potatoes that had been boiled for 3 minutes: long enough to be edible, but still hard to eat.
In three out of four groups, one chimp collected a stick and used it to mash the potato (). “They redevelop these kind of behaviours on their own from scratch,” says Tennie.
In a separate study involving Tennie, captive chimps figured out how to use sticks to dig up underground tubers, without ever having seen it done ().
Chimps find it easy to reinvent certain behaviours, says Kevin Langergraber of Arizona State University. The chimps he studies in Uganda rarely use tools to get food, but a few years ago there was a brief period when there were lots of insects living in fallen logs. The chimps quickly started using sticks to extract them. “It looked just like insight,” he says. “They weren’t there for 20 minutes trying it out. It’s just boom, there’s a log, there’s a stick, pick it up, there you go.”
However, Andrew Whiten of the University of St Andrews, UK, says the potato test could have been much easier than climbing a spiky palm tree and pounding its heart. If so, this wild behaviour may arise rarely and only spread by copying.
Tennie and his colleagues’ findings may help explain why our hominin ancestors kept making the same simple tools for millions of years. Like chimps, they might have repeatedly reinvented certain tools, but struggled to learn improvements from others.
Cultural variation, which leads to rapid improvements, only became prominent in the past few hundred thousand years – perhaps because our species was better at imitation than other hominins.
