Wild Chimpanzees Keep Hurling Rocks at the Same Trees—Now Scientists Are Trying to Figure Out Why

In Boé National Park, researchers have found that wild chimpanzees in the region throw rocks at the same trees for over a decade. The behavior, called accumulative stone throwing, has been observed in only four chimpanzee populations in West Africa. The researchers, including Robyn Nakano and Ammie Kalan from the University of Victoria, documented this activity at sites where trees have been marked by gouged bark and piles of stones. The video recordings from the Pan African Programme show that mostly adult male chimpanzees throw rocks at specific trees, often accompanied by pant-hooting and buttress drumming. These actions suggest that the behavior may be a cultural practice, not simply a response to environmental factors.

The team found that most sites first identified by the Pan African Programme and revisited in 2017 remained active when Nakano and Kalan’s team returned in 2026. The chimpanzees have returned to these same trees to throw rocks for more than ten years. The video and audio collected during this trip will allow the researchers to identify the age and sex of individual stone-throwers and observe how nearby chimpanzees respond to these events. This data could help determine the purpose of the behavior. The researchers also noted that the behavior occurs in a social context rather than during extractive foraging, differing from other primate behaviors.

Chimpanzees’ habitat in Guinea-Bissau is becoming increasingly threatened by industrial mining. During fieldwork, the researchers discovered boreholes dug to explore bauxite, the ore used to produce aluminum. Although bauxite mining might help promote economic development, comparable operations in neighboring Guinea have caused habitat loss and pollution, with negative effects on wildlife and local communities. The country’s political instability also presents challenges for environmental oversight. The researchers emphasize that losing these stone-throwing sites would be both a cultural and ecological loss, erasing a documented example of primate material culture. The purpose of the behavior remains unclear, but the evidence collected by the cameras and recorders could help determine whether the behavior functions as territorial signaling, long-distance communication, or something else entirely.

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