The use of stone tools by early hominins is considered a milestone in human evolution that led to changes in dentition, hand morphology and brain size. But despite the obvious ecological significance of making and using early stone tools, it remains unclear how the necessary skills first emerged in naïve individuals.
A new study involving five captive orangutans aimed to shed some light on this question. The study, conducted by Alba Motes-Rodrigo from the University of Tübingen and colleagues, tested the spontaneous tool making and using capabilities of two male orangutans (Pongo pygmaeus) at Kristiansand Zoo in Norway, and three female orangutans at Twycross Zoo in the UK. None of the orangutans had been trained or previously exposed to any of the behaviors of interest to the researchers.
Previous research had proposed that the simplest scenario in which the use of stones as tools could have arisen may have been when an individual picked up a stone and used it to crush a hard object, such as a nut, against another hard surface, in order to access the contents. This is termed lithic percussion technology and involves mostly the use of rocks in a hammer-and-anvil manner.
The use of stone tools by orangutans in the wild has never been documented in the scientific literature. Although these apes are avid tool users, their tools usually consist of branches, sticks or other pieces of vegetation. Orangutans spend most of their time in the canopy where stones are not available. Therefore, orangutans can be considered naïve in terms of exposure to the use of stones as tools.
In the first part of the current study, the two male orangutans were given a concrete hammer, a prepared stone core from which flakes could be struck, and two puzzle boxes requiring the apes to cut through a rope or a silicon skin in order to access a food reward. Both orangutans spontaneously hit the hammer against the walls and floor of their enclosure, but neither directed strikes towards the stone core.
In the next part of the study, the orangutans were also given a human-made sharp flint flake, which one orangutan used to cut the silicon skin, thereby opening the puzzle box. According to the researchers, this is the first demonstration of cutting behavior in untrained, unenculturated orangutans.
The three female orangutans from Twycross Zoo had also never been exposed to the production or use of stone flakes before. They were given demonstrations by the researchers on how to strike a core in order to create a flint flake. In this way, the researchers assessed whether the apes could learn this skill by observing others. After the demonstrations, one female went on to use the hammer to hit the core, directing the blows towards the edge as had been demonstrated.
The study is the first to report spontaneous stone tool use without close direction in orangutans that have not been enculturated by humans. The authors say their findings suggest that the last common ancestor between orangutans and humans probably had the cognitive and physical skills to engage in lithic percussive behavior, as demonstrated by the captive orangutans in this study.
In addition, the fact that these orangutans spontaneously struck with stone hammers and recognized sharp stones as cutting tools shows that two major prerequisites for the emergence of early stone tool use are present in these apes, and may have existed in our last common ancestor with orangutans, 13 million years ago.
“Our study is the first to report that untrained orangutans can spontaneously use sharp stones as cutting tools,” said the researchers. “We also found that they readily engage in lithic percussion and that this activity occasionally leads to the detachment of sharp stone pieces.”
The study is published in the journal PLOS ONE.