A tree you'd walk past, if you didn't know what to look for

Walk through the savanna-woodland of Boé National Park in Guinea-Bissau and you might pass a tree covered in gnarled scars, its base ringed with a scattered pile of rocks, and think nothing of it. Researchers who study wild chimpanzees know better. That scarred trunk and stone pile mark one of the rarest documented behaviors in the primate world: accumulative stone throwing, in which chimpanzees repeatedly hurl or bang rocks against specific trees, building up conspicuous stone accumulations over years, sometimes decades.

Field researchers Robyn Nakano and Ammie Kalan, writing for The Conversation this week after returning from a recent expedition to the site, reported something that speaks directly to how durable this behavior actually is: most of the accumulative stone throwing sites first identified by the Pan African Programme and revisited by their team in 2017 were still in active use during their most recent trip. That means individual chimpanzees, or at minimum the local tradition itself, have kept returning to the exact same trees for more than ten years.

A behavior that shouldn't, by most logic, exist at all

Stone tool use in chimpanzees isn't itself unusual; wild chimps in various regions are well documented cracking nuts with hammer stones. What makes accumulative stone throwing genuinely strange is that it serves no obvious practical purpose. The chimpanzees aren't extracting food or processing anything edible. They're picking up a rock, throwing or banging it against a tree trunk, and often pant hooting, a loud, long-distance vocal signal, while doing it. Some individuals combine the stone throwing with buttress drumming, hitting their hands and feet against the tree in a rhythmic display.

Researchers first documented the behavior in 2016 across four chimpanzee communities scattered through Guinea, Guinea-Bissau, Liberia, and Ivory Coast, using camera traps positioned near stone piles that field teams had stumbled across during routine surveys. A decade later, the behavior has still only been confirmed in those same four populations, out of dozens of chimpanzee research sites studied across the African continent. That narrow, patchy distribution is itself a key piece of evidence: if the behavior were simply an inevitable byproduct of chimpanzees having access to rocks and trees, it should show up far more widely than it does.

Why researchers think this counts as culture

The prevailing theory holds that accumulative stone throwing represents a modified version of an already-known chimpanzee display behavior: the combination of pant hooting and buttress drumming that adult males use to communicate over distance and assert presence. Under this framework, the stone throwing may function as a kind of amplified or specialized version of that ordinary display repertoire, rather than an entirely separate invention.

But researchers haven't ruled out something closer to symbolic meaning. Previous work has suggested the stone accumulation sites may mark significant locations within a chimpanzee community's territory, not unlike how some Indigenous communities in West Africa maintain stone collections at trees they consider sacred. Nakano and Kalan describe the behavior as likely cultural specifically because of that limited, patchy distribution: the mere presence of rocks and appropriate trees doesn't guarantee a site will emerge, meaning the practice has to be learned and transmitted socially within a community rather than arising automatically wherever the right materials exist. That's the same standard researchers use to distinguish genuine animal culture from behavior that's simply a universal response to shared environmental conditions.

The fieldwork behind the finding

Getting this data required a genuinely difficult expedition. The Boé chimpanzees aren't habituated to humans, meaning researchers can't follow or observe them directly on foot without the animals fleeing. Instead, the team cycled and hiked 22 kilometers from their base village of Béli into the savanna-woodland to establish a remote bush camp, working alongside local field assistants and a graduate student from the University of Victoria's Great Ape Behaviour Lab. From there, they set up paired video cameras at each known stone-throwing site and positioned separate audio recorders nearby to capture vocalizations from chimpanzees who might be responding to the display, not just performing it.

That audio component matters because it's aimed at a question the original 2016 discovery couldn't answer: what these sites actually communicate, and to whom. By recording who throws stones, along with the age, sex, and behavior of any other chimpanzees within earshot, researchers hope to determine whether accumulative stone throwing functions primarily as a mating display, a territorial marker, or something closer to a ritualized social signal, distinctions that matter for understanding how complex, non-instrumental communication first emerged in a lineage that eventually produced human symbolic behavior, a question that connects to broader debates already reshaping how researchers classify our closest fossil relatives.

A cultural record under active threat

The persistence of these sites over a decade or more isn't just a scientific curiosity; it's the reason researchers are treating their preservation as an urgent conservation issue in its own right. Accumulative stone throwing represents a form of durable, non-food-related material culture in a non-human species, and losing it, whether through habitat destruction or population decline, would mean erasing a genuine piece of primate cultural heritage that took years, possibly generations, to establish.

That risk isn't hypothetical. While in the field, Nakano and Kalan's team encountered active bore holes from bauxite mining exploration inside chimpanzee habitat. Bauxite mining offers real economic development potential for Guinea-Bissau, one of the world's poorer nations, but the same industry has already caused documented habitat destruction and pollution in neighboring Guinea, with direct consequences for local chimpanzee populations. Guinea-Bissau's own history of unstable governance compounds the risk, since consistent environmental oversight is difficult to sustain even when regulations exist on paper.

What a decade of returning tells researchers

The simplest fact in this story may end up being the most consequential one. A behavior first documented by chance, at a handful of sites, in a single research window a decade ago, has now been confirmed still active on a return visit. That kind of longitudinal confirmation is rare in primate field research, where funding gaps and access difficulties often mean researchers document a phenomenon once and never manage a proper follow-up. Every additional year these specific trees stay in use adds real weight to the argument that accumulative stone throwing isn't a fluke or a temporary fad, but a genuine, stable cultural tradition, the kind that, once lost, doesn't simply reappear somewhere else.