Walking through the savanna-woodland landscape of Boé National Park, Guinea-Bissau, you might encounter a tree covered in gnarled scars, with an accumulation of rocks surrounding its base. The chimpanzees may have left the area, but you are lucky nonetheless, because you have stumbled upon evidence of a rare — and potentially cultural — chimpanzee behavior: accumulative stone throwing. Video recordings show wild western chimpanzees, usually adult males, throwing rocks at specific trees and repeatedly returning to these trees to perform the behavior.
While throwing, the chimpanzees pant hoot — a loud, long-distance communicative signal — and sometimes repeatedly hit their hands and feet on the tree in a behavior called buttress drumming. We have just returned from a field site in Guinea-Bissau where we collected data to help us investigate the social and ecological context of accumulative stone throwing to determine what these chimpanzees are trying to communicate. Given our evolutionary relatedness to chimpanzees, we hope accumulative stone throwing can help us understand the emergence of complex communication and stone tool use over the course of human evolution.
Pant hooting and buttress drumming are both part of the male chimpanzee display, suggesting that accumulative stone throwing might represent a modification of this common behavior. It is likely a cultural behavior due to its limited distribution, and because the availability of rocks and trees does not guarantee the presence of an accumulative stone throwing site. Previous research suggests that accumulative stone throwing is likely communicative or may even have a symbolic purpose, with sites marking important locations within the chimpanzees’ territory.
However, we still don’t know what accumulative stone throwing sites might mean to the chimpanzees themselves nor why they do it. While some primates use stone tools to access food, for instance to crack open nuts, accumulative stone throwing is a rare example of stone tool use in a social context. It has been observed in only four chimpanzee groups in West Africa to date.
We traveled to the remote Boé chimpanzee territory in Guinea-Bissau and based ourselves in Béli, a small village where, in collaboration with local people, the Dutch non-governmental organization Chimbo maintains a compound. Visiting researchers and tourists can stay here and use a workspace with solar-generated electricity. From Béli, we cycled and hiked 22 kilometers into the savanna-woodland to establish a bush camp with our two field assistants, Djei Baldé and Balu Séra, and a master’s student from the Great Ape Behaviour Lab, Taylor Tippett.
The Boé chimpanzees performing the behavior are unhabituated; they are not used to humans, meaning that we cannot observe individual chimpanzees on foot because they will run away. Instead, we collected behavioral data using camera traps and recording devices. We set up two video cameras at each accumulative stone throwing site and placed the recording devices strategically to capture audio data from the areas around these sites.
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