A strange behavioural phenomenon was recorded by a team of primatologists from the Max Planck Institute for Evolutionary Anthropology in Leipzig. The researchers, analyzing some groups of chimpanzees in the forests of West Africa, have noticed that some male specimens are usually throwing stones against the trunks of the trees and then screaming to run away. The behaviour seems to be so usual that the researchers themselves have noticed real piles of rocks at the base of these trees.
The researchers did not understand why this strange behavior and the only thing they found is that the chimpanzees themselves show that they prefer those trees that make more noise once the stone has been thrown. This led the researchers themselves to think that it is a form of communication or that the same sound of the stone thrown against the trunk gives them a kind of pleasure, a bit like when we listen to music.
Studying this behaviour is Ammie Kalan who, together with his team, has been trying since 2016 to find an explanation but without obtaining satisfactory results. The researcher thought that animals can use this loud noise to communicate with other members of the group in other areas of the forest, for example to demonstrate male dominance of a certain area.
She also organised an experiment with some French researchers to test the sound differences of different types of wood. The researcher, together with her colleagues, threw several stones of different sizes against 13 different types of trees and, apart from the fun of such an experiment, found only that the trees preferred by monkeys seem to be the ones with the most lasting and lowest sound. Among these trees is the treculia tree, a tree native to West Africa and Madagascar.
The researchers found this strange behaviour in four groups of chimpanzees, one in Guinea-Bissau and the others in Guinea, Liberia and Ivory Coast. The geographical limitation of this behaviour suggests that it is a kind of local cultural tradition of the different groups.