After watching all of the Planet of the Apes films, we noticed something disturbing: we never see primates playing rock-paper-scissors! And yet, they’re perfectly capable of playing. A recent study carried out by the University of Kyoto (Japan) and Beijing (China) shows that chimpanzees can learn to play this game and master it as well as a four year-old child. Read on to learn more about this discovery.

The study, published in the journal Primates, aimed to determine whether chimpanzees could learn and master a "transverse" task. Rock-paper-scissors is the perfect game because it implements so-called circular relationships between three elements: the flat hand (paper) covers the fist (rock) which breaks the two fingers (scissors) which in turn cut the paper. In this game (invented in China), the relationships are not always linear and the elements must be interpreted contextually because none of the elements has a constant value (the rock isn’t always the strongest, for example). It's important to note the role played by non-linear relationships in the lives of animals, who must sometimes choose elements according to context rather than by simple discrimination. In humans, we know that children can understand transverse patterns beginning at the age of 4.5 years. The researchers wanted to compare the chimpanzees’ learning processes to those of children.

For this study, Chloe, Casserole, Ayumu (the only male) and four of their peers were seated in front of a touch screen where they had to select the correct stimulus from the two presented to them. When the chimpanzees chose the right answer, they received a piece of apple. If they chose the wrong answer, a buzzer went off. During the training phase, the primates were trained on the relationships between the objects. When they were presented with photographs (hands of chimpanzees symbolizing the rock, the paper, and the scissors), they were progressively taught to choose the most powerful symbol in the configuration. The first pairs were paper-rock, the next were rock-scissors, followed by paper-rock again. After reaching 90% accuracy in each session, the chimpanzees participated in sessions that combined the three pairs. Once the training was over, the researchers found that five of the seven chimpanzees understood how the game worked after an average of 307 sessions. The results do however show that the animals had a difficult time finalizing the circular model, manifested by the fact that more paper-scissors sessions were need compared to the other sessions. Next, generalization tests were carried out with five other stimuli (photographs of human hands) that the chimpanzees quickly learned.

The same protocol was used with 38 children ages 2.9 to 5.9 years (average = 4.5 years; 17 girls and 21 boys). Unlike the chimpanzees, the children needed the same number of tries (5.8 on average) for all three pairs, presented independently. From around the age of four years, their accuracy during the mixed sessions increases substantially (and becomes very similar to the chimpanzees), which leads to the conclusion that children acquire the ability to solve transverse patterning problems at this age.

In the end, even if the chimpanzees needed more time to learn the game, their performance was similar to that of a four year old child.