Orangutans instinctively use hammers to strike and sharp stones to cut – study

Orangutans instinctively use hammers to strike and sharp stones to cut, a study has found.

According to the researchers, the findings suggest that two major prerequisites for the emergence of stone tool use – striking with stone hammers and recognising sharp stones as cutting tools – may have existed in our last common ancestor with orangutans, 13 million years ago.

Researchers tested tool making and use in two captive male orangutans that had not previously been trained or exposed to demonstrations of the activities.

Each animal at Kristiansand Zoo in Norway was provided with a concrete hammer, a stone, and two baited puzzle boxes.

They needed to cut through a rope or a silicon skin in order to access a food reward.

The study found the orangutans spontaneously hit the hammer against the walls and floor of their enclosure, but neither directed strikes towards the stone to create a sharp tool.

In a second experiment, the orangutans were also given a human-made sharp flint flake, which one orangutan used to cut the silicon skin, solving the puzzle.

Alba Motes-Rodrigo, at the University of Tubingen in Germany, and colleagues say this is the first demonstration of cutting behaviour in untrained orangutans.

In order to investigate if apes could learn the remaining steps from observing others, researchers showed three female orangutans at Twycross Zoo in the UK how to hit the stone to create a sharp flint flake.

After the demonstrations, one female went on to use the hammer to hit the stone, directing the blows towards the edge as demonstrated.

This study is the first to report spontaneous stone tool use without close direction in orangutans that have not been socialised by humans, researchers say.

Writing in Plos One, the researchers said: “When presented with a human-made flake, a naive orangutan spontaneously used it as a cutting tool to open a puzzle box, providing proof of concept that cutting (or piercing) using sharp-edged tools is within orangutans’ spontaneous repertoire.

“Overall, our findings suggest that two prerequisites for the emergence of early lithic technologies – lithic percussion (the removal of sharp stones) and the recognition of sharp-edged stones as cutting tools – might be deeply rooted in our evolutionary past.”