Wild chimpanzees have pelted the same trees with rocks for over a decade, and no one knows why

Deep in a remote stretch of West African forest, certain trees have accumulated piles of rocks at their base and gnarled scars on their bark, evidence of a behavior that wild chimpanzees have repeated at the same sites for more than ten years. Researchers who study the animals still cannot say what the ritual actually accomplishes, despite years of observation using camera traps and audio recorders. The behavior is rare enough, and specific enough, that it has become one of the more puzzling open questions in primate research.

Unlike the well-documented cases of chimpanzees using stones as tools to crack open nuts, this behavior does not appear to serve any obvious nutritional purpose. The animals are not extracting food or building anything with the rocks; they are simply throwing them at particular trees, again and again, often the same trees that other chimpanzees in the group have targeted before them.

A Rare Ritual Documented in Only a Handful of Groups

Scientists have identified this behavior, known as accumulative stone throwing, in only four known groups of wild western chimpanzees across West Africa, according to a summary of the research. In Boe National Park in Guinea-Bissau, researchers found trees covered in visible impact scars, with rocks piled up at the base from repeated strikes over the years. The sites are not one-off events; the same trees show evidence of use going back more than a decade, meaning generations of chimpanzees in the same community have returned to them repeatedly rather than the behavior being a single animal’s habit.

Pant-Hoots, Drumming, and Deliberate Aim

Observers describe the chimpanzees, usually adult males, approaching a target tree, throwing a rock at it, and often combining the act with a loud pant-hoot vocalization used for long-distance communication within the group. Some individuals also perform buttress drumming, repeatedly striking the tree’s broad root flares with their hands and feet, adding a second layer of sound to the display. The combination of a physical strike, a vocal call, and rhythmic drumming suggests the behavior is communicative in some sense, aimed at other chimpanzees who may be out of sight but within earshot, rather than being incidental or purely mechanical.

Studying Animals That Have Never Been Habituated to Humans

Much of what researchers know about the Boe population comes without the benefit of habituation, the years-long process by which primatologists typically get wild animals comfortable enough with human observers to be watched directly. Because these particular chimpanzees have not been habituated, researchers have relied on camera traps and remote audio recorders placed near known stone-throwing sites to document the behavior, piecing together patterns from indirect evidence rather than firsthand observation. That approach has been enough to confirm the behavior’s persistence over more than a decade, even without researchers ever standing close enough to watch a full sequence unfold in person.

A Genuine Open Question in Primate Behavior

Scientists studying the phenomenon are candid that they do not yet know what function it serves. It does not appear to be tool use in the conventional sense, since the chimpanzees are not using the rocks to access food, and it does not map neatly onto known territorial marking or mating displays documented in other primates. Researchers note that accumulative stone throwing represents a rare instance of stone-related behavior occurring in a purely social context rather than a foraging one, which is part of what makes it stand out. For now, the sites keep accumulating rocks, the behavior keeps recurring across chimpanzee generations, and the explanation remains one of the more open questions left in the study of great ape behavior.

Competing Ideas, None Yet Confirmed

Researchers studying the behavior have floated several possible explanations without being able to confirm any of them. One idea treats the throwing and drumming as a long-distance signal broadcasting a male’s location, strength, or presence to other members of the community who may be scattered across dense forest terrain where visual contact is impossible. Another possibility ties the sites to some form of territorial marking, similar in spirit to how other animals mark boundaries, though the evidence for that interpretation remains thin. A third line of thinking treats the accumulated scars and rock piles almost like a landmark, a location repeatedly reinforced by successive individuals for reasons that may be more about tradition within the group than about any single practical function.

Why This Behavior Matters Beyond One Species

Documenting a behavior this specific, this consistent, and this long-lasting in a wild, non-habituated population is itself a meaningful contribution to primatology, regardless of whether researchers ever pin down its precise purpose. Behaviors that persist across chimpanzee generations at the same physical locations are often treated as candidate examples of animal culture, meaning they are learned and passed down within a community rather than being instinctive behavior every chimpanzee performs regardless of upbringing. If accumulative stone throwing turns out to fit that pattern, it would add to a small but growing body of evidence that great apes maintain distinct local traditions, shaped by social learning, in much the same way human communities pass down customs that outlast any single person who practices them.

This article was produced with AI assistance and edited by Morning Overview staff.

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