Why Do Wild Chimpanzees Throw Stones At Trees?

Enlarge this image toggle caption Courtesy MPI-EVA PanAf/Chimbo Foundation Courtesy MPI-EVA PanAf/Chimbo Foundation

On Monday, a team of 80 people led by Hjalmar S. Kuhl and Ammie K. Kalan published an open-access paper in Nature's "Scientific Reports" that describes never-before-seen stone-throwing behavior among wild chimpanzees in four West African populations.

The chimpanzees throw the stones at trees or right into tree cavities. Because the apes reuse the tree sites and the accumulated stones — and because their behavior is not related to foraging — the researchers raise the possibility that this behavior is bound up with a type of ritual never before documented in wild apes.

Primatologists have known for a long time that chimpanzees in some populations use stones as hammers to crack open hard-shelled nuts or as cleavers to force open fruits, or throw stones in male displays. What's new about the stone tool-using behavior described this week — which the primatologists call "accumulative stone throwing" — is that it's completely divorced from any context related to foraging and leads to what essentially becomes an archaeological site. Judging by the wear on the trees from the stones' impact, the tree sites have been regularly used over some period of time.

I am delighted that the paper is open-access, which allows anyone interested to dive right in, read the details, and watch the short videos offered at the conclusion of the text. Here are some of the basics, taken from the paper:

"At four [research sites]: (Boé, Guinea-Bissau; Sangaredi, Guinea; Mt. Nimba, Liberia; and Comoé GEPRENAF, Côte d'Ivoire) we found multiple hollow and/or buttressed trees exhibiting clear signs of wear with an accumulation of rocks at their base or inside the tree. Using remote video camera traps, we subsequently filmed chimpanzees at each of these four [sites] approaching focal trees with a stone in their hand, or grabbing a stone from the base or from inside the tree's hollow cavities, and then proceeding to throw it (N = 64 total stone throwing events)."

toggle caption Courtesy of MPI-EVA PanAf/Chimbo Foundation

No observations of similar behavior have been made at the longest-term chimpanzee field sites, including Jane Goodall's famous Gombe. The big question, of course, is why are these chimpanzees — mostly, but not always, adult males in full-on, pant-hooting display mode — hurling stones in this specific way?

Kuhl, Kalan and their co-authors offer two possibilities. Perhaps the behavior originated as a kind of super-sized, noisy male display and now sometimes occurs outside that context. Or, the chimpanzees could be participating in a symbolic ritual. The authors write:

"Marking territorial boundaries and pathways with cairns has been an important practice of many historical human societies. For example, stone accumulation shrines at 'sacred' trees are well described for indigenous West African peoples. Superficially, these cairns appear very similar to what has been described here for chimpanzee accumulative stone throwing sites, thus it would be interesting to explore whether there are any parallels between chimpanzee accumulative stone throwing and human cairn building, especially in regions of West Africa where the local environment is similar."

The comparison between practices of chimpanzees and of West African peoples is inappropriate, says Alex Golub, sociocultural anthropologist at the University of Hawaii, Manoa and writer at the Savage Minds blog. Golub told me by email on Tuesday:

"I think comparing chimps and Africans this way sounds kind of racist. But setting aside its politics, it also strikes me as silly — didn't we figure out in the 19th century that like phenomena could have different causes? The idea that two practices, decontextualized and unexplained, might be suitable for comparison strikes me as backwards. We need to better understand these behaviors, their contexts, and what produced them rather than start up with Victorian ethnographic analogy."

The very definition of "ritual" is hotly contested (as the authors of the paper note). Golub believes a stronger case for ritual could be made if researchers saw "coordinated movement amongst multiple chimps in the course of a single stream of interaction, and then saw that coordinated movement happen again at the same spot repeatedly (multiple instantiations of the same ritual 'script')" — without the observed actions being related to mating or food.

Anthropologist and primatologist Jill Pruetz of Iowa State University, known for her discoveries of spear hunting by chimpanzees in Senegal and a co-author of the stone-throwing paper, acknowledged to me that what counts as ritual is hotly debated, but in an email on Monday conveyed her excitement about the stone-throwing discoveries:

"[It is] the accumulation of stones [that] I find to be utterly surprising and it once again demonstrates that there is so much more to learn about chimpanzees, which I always note are probably the most intensively studied wild animal! It is one of those behaviors that I don't think you could have predicted (like hunting bushbabies with 'spears' at my site!), but one that is really fascinating. Other animals cache objects, so we shouldn't find it so difficult to accept, but I think that precisely because it is chimps doing it, many people will have a hard time coming to terms with the behavior."

My take? I think the stone-hurtling together with the caches at the trees does open up fascinating new avenues of thinking and hypothesis-testing about these close primate relatives of ours.

What gives me great pause is any suggestion, as Laura Kehoe, one of the paper's co-authors, made on Monday, that we're dealing with "sacred" rituals by these chimpanzees. I can think of no good scientific reason to indulge in speculation that chimpanzees would have any notion of the sacred.

Barbara J. King is an anthropology professor at the College of William and Mary. She often writes about human evolution, primate behavior and the cognition and emotion of animals. Barbara's most recent book on animals is titled How Animals Grieve. You can keep up with what she is thinking on Twitter: @bjkingape.