Researchers Study Why Chimpanzees Repeatedly Throw Stones at Specific Trees

In Guinea-Bissau’s Boé National Park, certain trees bear repeated marks from stones thrown by chimpanzees, with piles of rocks often found at their bases. This accumulative stone throwing behavior, mainly observed among adult male chimpanzees, involves hurling stones at particular trees and returning to the same spots multiple times.

During these displays, chimpanzees emit loud pant-hoot calls that can travel long distances and sometimes engage in buttress drumming by striking the tree with their hands and feet. Researchers recently returned from fieldwork in Guinea-Bissau, collecting new data to better understand the ecological and social context of this behavior and its possible communicative function.

Investigating the Cultural Significance

Accumulated stone throwing is considered a cultural behavior because it appears only in certain chimpanzee groups despite the availability of rocks and trees elsewhere. It likely represents a modification of typical male chimpanzee displays, which include pant hooting and buttress drumming. Prior studies suggest these stone-throwing sites may mark important locations within chimpanzees’ territories and serve communicative or symbolic purposes.

Unlike other primate uses of stone tools, such as cracking nuts for food, accumulative stone throwing occurs in a social context and has been documented in only four West African chimpanzee groups. The exact meaning and significance of these sites to the chimpanzees remain unknown.

Fieldwork in Boé National Park

The research team based themselves in Béli village, collaborating with local residents and the Dutch NGO Chimbo, which maintains a compound equipped with solar-powered electricity for researchers and visitors. From there, they traveled 22 kilometers into the savanna woodland to establish a bush camp with field assistants and a master’s student from the Great Ape Behavior Lab.

Because the Boé chimpanzees are unhabituated and avoid humans, direct observation was not possible. Instead, the team used camera traps and audio recorders placed at accumulative stone throwing sites to gather behavioral data. Two video cameras were set up at each site, along with strategically positioned recording devices to capture sounds around the trees.

Their campsite was near the Fefine River, a vital water source in the dry savanna-woodland environment. Cameras also recorded local wildlife near the riverbank.

Data Collection and Observations

Each day, the researchers visited two to five sites to maintain equipment, collect data, measure trees, and perform 3D scans of stones thrown at the trees for further analysis. Along their routes, they noted chimpanzee nests, feeding signs, vocalizations, and occasional sightings.

The collected video and audio recordings will help determine the social characteristics of accumulative stone throwing, including the age and sex of the individuals throwing stones and the presence of other chimpanzees nearby who might respond. This information aims to clarify what chimpanzees communicate through this behavior.

Sites initially identified by the Pan African Program and revisited by the team in 2017 were still active during the recent fieldwork, indicating that chimpanzees may use these locations for over ten years.

Conservation Concerns

As many primate species face threats from human activities, preserving cultural behaviors like accumulative stone throwing is important for maintaining their adaptive capacity and cultural heritage. This behavior represents durable primate material culture, and its loss would mean erasing an aspect of primate history.

Chimpanzee habitats in Guinea-Bissau are endangered by industrial activities, particularly bauxite mining exploration, which the researchers encountered during their fieldwork. While bauxite mining offers economic development opportunities, it also risks habitat destruction and pollution, negatively impacting chimpanzees, other wildlife, and local communities, as observed in neighboring Guinea.

Effective environmental oversight and regulation are necessary, especially given Guinea-Bissau’s governance challenges. By studying and highlighting chimpanzee cultural behaviors such as accumulative stone throwing, researchers aim to support conservation efforts, biodiversity preservation, and the safeguarding of primate cultural materials for future study and education.