In the savanna-woodland of Boé National Park, Guinea-Bissau, trees bear unmistakable scars—gnarled bark and accumulations of stones at their bases—left by wild chimpanzees over more than a decade. These weathered marks are evidence of a behavior so rare that scientists have traveled thousands of kilometers to understand it: accumulative stone throwing, a phenomenon observed in only four chimpanzee groups across West Africa.
Adult male western chimpanzees have been recorded repeatedly returning to the same trees to throw rocks, often accompanying the throws with pant hoots—loud, long-distance calls—and buttress drumming, where they strike their hands and feet against the trunk. This is not random destruction. Video recordings reveal a persistent, purposeful pattern: males target specific trees and maintain these sites over years, suggesting something far more complex than simple play or territorial aggression.
Researchers from the Max Planck Institute of Evolutionary Anthropology and collaborating institutions believe this behavior is cultural, passed down within groups rather than driven by the mere availability of rocks and trees. If rocks and suitable trees were the only requirements, accumulative stone throwing would appear throughout chimpanzee populations where such materials exist. It doesn't. Its narrow distribution among West African groups points to something learned, shared, and meaningful within those communities—a hallmark of culture.
To understand what chimpanzees are trying to communicate through this behavior, a team of researchers established a remote bush camp 22 kilometers into the Boé territory, working with local field assistants Djei Baldé and Balu Séra and collaborating with the Dutch organization Chimbo. Because the Boé chimpanzees are unhabituated—not accustomed to human presence—researchers could not follow individuals on foot. Instead, they deployed camera traps and recording devices strategically positioned at accumulative stone throwing sites, capturing video and audio data while maintaining distance from the animals. They also documented chimpanzee nests, feeding signs, and vocalizations, collected precise measurements of targeted trees, and created 3D scans of the thrown rocks for later analysis.
The data collected will allow researchers to examine the social context of each throwing event: the age and sex of the stone thrower, whether other chimpanzees were present as an audience, and how they reacted. These details matter because they help answer a pressing question: What are these chimpanzees trying to communicate? Previous research suggests accumulative stone throwing sites may mark important locations within chimpanzee territories, functioning as symbolic landmarks. The behavior might even serve a communicative purpose beyond immediate social display—a form of stone tool use in a genuinely social context, distinct from the well-known practice of using stones to crack open nuts.
For human evolution specialists, the significance runs deeper. Given our shared ancestry with chimpanzees, understanding accumulative stone throwing offers a window into how complex communication and stone tool use emerged in our own species. The behavior represents something between tool use and language: purposeful, repeated, socially embedded, and potentially meaningful in ways we are only beginning to decode. The scarred trees of Boé National Park, maintained by chimpanzees for over a decade, remind us that culture and communication are not uniquely human—they are older than our species.