West Africa chimpanzees have been observed conspicuously throwing stones at trees and "hooting" as they tumble down into a pile either near or in the hollow of a tree. Researchers believe this never-before-seen behavior likely has some cultural roots and may even be the animals' way of marking "sacred trees."
Scientists from the Max Planck Institute for Evolutionary Anthropology made the discovery with the help of camera traps set up after piles of stones were found at several of their research sites. Their research is part of the Pan African Programme: The Cultured Chimpanzee (PanAf), which is aimed at understanding the ecological and evolutionary drivers of behavioral diversification in chimpanzees.
"The PanAf cameras filmed individual chimpanzees picking up stones from beside, or inside trees, and then throwing them at these trees while emitting a long-distance pant hoot vocalization," study co-author Ammie Kalan said in a news release.
These ritualized displays were exhibited primarily by adult male chimps, however, researchers noted that some females and juveniles were also seen throwing stones at trees.
"This study reports a new chimpanzee behavior not known previously and highlights the potential of the PanAf project to uncover unknown facets of the life of chimpanzees, our closest living relative," explained Christophe Boesch, director of the Department of Primatology at the Max Planck Institute for Evolutionary Anthropology.
Chimpanzees are proficient tool users, able to use sticks to reach bugs at honey from the and stone or wooden hammers to crack open nuts. However, this behavior doesn't seem to have anything to do with foraging.
"As the stone accumulation behavior does not seem to be linked to either the abundance of stones or the availability of suitable trees in an area, it is likely that it has some cultural elements," Boesch added.
Therefore, the exact reasoning by this unfamiliar behavior is still open to debate. On theory is that the chimpanzees are perhaps marking sacred trees.
Nonetheless, researchers note their findings raise questions regarding the interpretation of stone accumulation found at archaeology sites and may even shed light on the origin of ritual practices in hominin evolution.
Their study was recently published in the journal Scientific Reports.
