According to a study published February 16 in the open-access journal PLOS ONE by Alba Motes-Rodrigo of the University of Tübingen in Germany and colleagues, untrained captive orangutans can complete two major steps in the sequence of stone tool use: striking rocks together and cutting with a sharp stone.
Two captive male orangutans (Pongo pygmaeus) were used to test tool manufacturing and usage at Kristiansand Zoo in Norway. Neither had been previously taught or shown examples of the goal behaviour. Each orangutan received a concrete hammer, a prepared stone core, and two baited puzzle boxes in which they had to chop through a rope or a silicon skin to get access to a food reward. Both orangutans spontaneously smashed the hammer on the enclosure’s walls and floor, but neither struck the stone core. In a separate experiment, orangutans were given a sharp flint flake created by humans, which one orangutan used to cut the silicon skin and solve the riddle. This is the first time cutting behavior has been seen in orangutans that have never been taught or enculturated.
The researchers then showed how to hit the core to generate a flint flake to three female orangutans at Twycross Zoo in the United Kingdom to see whether they could learn the following stages from watching others. Following these examples, one female proceeded to smash the core with the hammer, guiding the blows towards the edge as shown.
This is the first time that orangutans that have not been enculturated by humans have used stone tools without being directed. Our last common ancestor with orangutans, 13 million years ago, may have had two fundamental conditions for the advent of stone tool use: hitting with stone hammers and identifying sharp stones as cutting tools, according to the scientists.
The authors continue: “Our research is the first to show that untrained orangutans can utilize sharp stones as cutting tools on their own. We also discovered that they like lithic pounding, which sometimes results in the separation of sharp stone fragments.”
