Chimpanzees love to throw stones at trees

Studying chimpanzee behaviour across Africa, researchers have discovered a previously unknown aspect of man's closest living relatives: they love to throw stones at trees.

While the researchers do not yet know why exactly the animals do this, they believe that the behaviour has some cultural elements.

The findings were published in the journal Scientific Reports.

The study "Pan African Programme: The Cultured Chimpanzee" (PanAf) aims to better understand the ecological and evolutionary drivers of behavioural diversification in chimpanzees.

Following a unique standardised protocol, data on chimpanzee behaviour, demography and resource availability has been collected since 2010 at 39 different temporary research sites across Africa.

"The PanAf project represents a new approach to studying chimpanzees and will provide many interesting insights into chimpanzee demography and social structure, genetics, behaviour and culture", said one of the researchers Hjalmar Kuehl from Max Planck Institute for Evolutionary Anthropology, Germany.

After discovering conspicuous piles of stones next to trees at four research sites in West Africa, the field teams placed camera traps next to them.

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 vocalisation.

Importantly, the behaviour results in accumulations of rocks at these locations.

Whereas it is mainly the adult males practicing this behaviour in the context of ritualised displays, some camera traps also revealed females or juveniles doing it.

The behaviour has only been observed in West Africa and appears to be independent of any foraging context, in which the majority of tool-use behaviours were previously described in chimpanzees.

"This study reports a new chimpanzee behaviour not known previously and highlights the potential of the PanAf project to uncover unknown facets of the life of chimpanzees, our closest living relative," Christophe Boesch, director of the Department of Primatology at the Max Planck Institute for Evolutionary Anthropology, noted.

"As the stone accumulation behaviour 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 noted.