Chimpanzees in West Africa have been exhibiting an intriguing behavior for more than ten years: the repetitive habit of throwing rocks at specific trees, accumulating piles of rocks at their bases. Scientists consider this activity a potential cultural ritual, which may offer clues about the evolution of communication and tool use among primates, including humans. The phenomenon was first observed on August 28, 2026, and remains a fascinating mystery to science.
The enigmatic stone throwing by primates
The practice, known as “stone hoarding”, has been recorded in only four groups of wild chimpanzees to date. Observations show mainly adult males throwing rocks against tree trunks, returning to the same locations repeatedly. While throwing, they emit vocalizations called “pant hoots” – a long-range communication signal – and sometimes beat their hands and feet against trees in a gesture known as “buttress drumming”. This behavior suggests intentionality beyond mere chance.
This type of display is part of the males’ communication repertoire, suggesting that stone throwing may be a variation of an already existing and adapted behavior. The scarcity of such places, even where stones and trees are available, reinforces the idea that it is a specific cultural manifestation of certain groups, transmitted between individuals.
Researchers unravel the context of behavior
To understand the meaning behind this ritual and what chimpanzees are trying to communicate, researchers from the Great Ape Behavior Lab traveled to Boé National Park in Guinea-Bissau. This remote territory is inhabited by wild chimpanzees unaccustomed to human presence, which means direct observations are impossible as the animals would flee. The team, which included field assistants Djei Baldé and Balu Séra and master’s student Taylor Tippett, set up a camp in the middle of the savannah forest, 22 kilometers from the village of Béli, where the Dutch NGO Chimbo maintains a support base.
At the camp, on the banks of the Fefine River – a crucial source of water in the savannah-forest landscape – the team adopted non-invasive methods to collect data. Waking up around 6:30 am, the researchers headed to the launch sites to maintain equipment and collect additional data.
- Video cameras: Two cameras were installed at each throwing site to record the chimpanzees’ action.
- Audio devices: Recorders were strategically positioned to capture sounds and vocalizations around the sites.
- Physical data collection: Regularly, SD cards and batteries were replaced. Tree measurements and 3D scans of thrown rocks were taken for detailed analysis.
- Additional observations: During their movements, the team recorded chimpanzee nests, feeding signs, vocalizations and sightings, creating a more complete picture of the ecosystem.
These data will make it possible to investigate social characteristics of behavior, such as the age and sex of the chimpanzees involved and the reaction of other nearby individuals. The researchers confirmed that most of the sites originally identified by the Pan African Program and revisited in 2017 are still in use, demonstrating that the habit has persisted for more than a decade.
The relationship with human communication and the use of tools
This unique cumulative stone throwing behavior is a rare example of stone tool use in a social context among primates. Unlike other known uses, such as cracking nuts for food, the exact purpose of stone throwing is not yet fully understood by scientists. The practice stands out for being an interaction with the environment that does not directly aim to obtain food.
Despite this, the evolutionary proximity between humans and chimpanzees suggests that studying this practice can provide crucial insights into the emergence of complex communication and the development of tool use throughout human evolution. Some scientists speculate that the throwing sites may have a symbolic purpose, marking important points in the chimpanzees’ territory or serving as a form of territorial signaling.
Environmental threats and the conservation of primate culture
The persistence of cultural behaviors is vital for the ability of primate species to adapt to environmental changes and for their own conservation. Understanding and protecting these cultural manifestations is essential in the face of growing pressures. However, chimpanzee habitat in Guinea-Bissau faces serious threats, mainly due to the expansion of industrial bauxite mining.
Bauxite exploration, although it represents a significant opportunity for Guinea-Bissau’s economic development, carries serious risks. It can result in habitat devastation, soil and water pollution, with profoundly harmful impacts on chimpanzees, other wildlife species and local communities – a scenario already observed in neighboring Guinea. The absence of effective environmental oversight and robust regulations further worsens the situation, especially in a context of unstable governance in the region.
Studying and highlighting these complex cultural behaviors, such as cumulative rock throwing, is crucial to strengthening chimpanzee conservation efforts. This contributes to the protection of biodiversity and the preservation of valuable primate cultural heritage, ensuring that it is available for future research and education. The research acts as a wake-up call about the urgent need to balance economic development with the protection of the environment and wildlife, safeguarding not only species but also their rich cultures.
