A Culture With No Backup

Researchers from the Great Ape Behaviour Lab at the University of Victoria went to check whether the practice had survived. Robyn Nakano and Ammie Kalan, working with the conservation organisation Chimbo, hung camera traps and audio recorders at sites first catalogued by the Pan African Programme and revisited in 2017. They also measured the trees and made 3D scans of the rocks. The Boé chimpanzees are unhabituated — they have not learned to tolerate a human sitting nearby with a notebook — so the cameras do the watching. Most of the old sites were still in use.

The behaviour has a name, accumulative stone throwing, and a short history. Hjalmar Kühl and colleagues described it in Scientific Reports in 2016, after a continent-wide camera survey turned it up in four West African chimpanzee groups and, across everywhere else the survey looked, nowhere. What happens is this: an adult male approaches a tree, usually one with heavy buttress roots, pant-hoots — the long-distance call that carries through forest — sometimes drums on the buttress with hands and feet, and throws a rock at it. Then leaves. The rock stays. Someone else, months later, throws another one.

The distribution is the first thing worth noticing. Rocks are everywhere. Buttressed trees are everywhere. The behaviour is in four groups. Availability doesn't explain it, which means it isn't being independently rediscovered by each population that happens to have the raw materials — it's being passed between individuals. That is the working definition of culture, and here it is doing structural work rather than serving as a compliment.

The second thing worth noticing is where the culture is stored.

Consider what the pile does. Every throw leaves a physical mark that makes the site more conspicuous, which makes the next chimpanzee more likely to recognise it as a site, which produces the next throw. The behaviour doesn't need to be held continuously in anyone's head. It is held in the ground. Biologists have a word for this arrangement — stigmergy, coined for termites, whose mounds are built by each insect responding to the deposits left by the last, with no termite anywhere holding the plan. I should be clear that nobody in this study proposed stigmergy; I am the one drawing the line. But the shape fits, and the shape has teeth.

Because it cuts both ways. The accumulation is the feature that makes these sites feel most like ritual to a human observer: a marked place, returned to, added to, across generations. It is also the feature that makes ritual unnecessary as an explanation. If the pile recruits the throw, the loop can run without meaning anything to anyone inside it. The most evocative fact about these trees is the same fact that most undercuts reading them as symbolic.

Except the distribution won't let the deflationary story stand either. A purely self-sustaining physical loop should ignite wherever rocks and buttressed trees coincide, which is most of the range. It doesn't. Four groups. Something has to start the loop, and whatever that is travels socially and unevenly, the way a practice does.

Nakano and Kalan decline to close the gap, which is the correct move and a rarer one than it should be. Their most defensible reading is modest: the throwing looks like an elaboration of the ordinary male display repertoire — pant-hoot, buttress drum, now with a stone for volume — possibly marking locations that matter within a territory. Beyond that, they say plainly that they still do not know what the sites mean to the chimpanzees, or why they do it.

Which leaves two things that aren't ambiguous at all.

The first is why we know about this at all. We know it because the behaviour happens to deposit rock. Something that piles up is something a camera trap can be aimed at and a survey can count. Four groups is a detection result, not a census — and every chimpanzee practice that leaves nothing behind has no discovery mechanism whatsoever. A habit of holding a stem a particular way, a route through a particular ravine, a preference passed down for twenty years: invisible to the method, and therefore absent from the ledger of what we are prepared to call culture. The pile is not only where this behaviour is stored. It is the reason it is on the list.

The second is what happens if the tree goes.

Human culture is redundant by construction. Burn a library and the practice survives in another library, another city, another head. Here the redundancy is thinner, and it is worth being exact about which part has a copy and which doesn't. The practice has four — four groups, all in one corner of West Africa, all under the same regional pressures. A culture with three backups, every one of them stored in the building that is on fire, is a more alarming object than a culture with none, because it is arithmetic rather than rhetoric.

The accumulation has no copy at all. The record of a decade of returning to this tree is the heap at its base, and that heap sits in one park, on ground where bauxite exploration is under way. Clear the tree and the pile is not damaged or interrupted; it is finished, and with it whatever the loop needed to keep running.

And then there is the other half, the half that lives in the animals. Something social has to start the loop, which means the transmissible part of this is stored in a population, not in a place. Mining does not only take ground; it displaces the individuals holding that half. Take the tree and the archive is gone. Take the chimpanzees and there is nobody left who knows to start another one — no neighbouring group to relearn from, because the nearest one that does this is not near. That is the version where the phrase stops being rhetorical.

Ten years of returning to one place is a long time for an animal that writes nothing down. It is not long at all measured against a mining lease.