Nothing like this had been seen before and it gave me goose bumps. Jane Goodall first discovered wild chimps using tools in the 1960s. Chimps use twigs, leaves, sticks, and some groups even use spears in order to get food. Stones have also been used by chimps to crack open nuts and cut open large fruit. Occasionally, chimps throw rocks in displays of strength to establish their position in a community.

But what we discovered during our now-published study wasn’t a random, one-off event, it was a repeated activity with no clear link to gaining food or status—it could be a ritual. We searched the area and found many more sites where trees had similar markings and in many places piles of rocks had accumulated inside hollow tree trunks—reminiscent of the piles of rocks archaeologists have uncovered in human history.

Videos poured in. Other groups working in our project began searching for trees with tell-tale markings. We found the same mysterious behavior in small pockets of Guinea Bissau, Liberia, and Côte d’Ivoire but nothing east of this, despite searching across the entire chimp range from the western coasts of Guinea all the way to Tanzania.

Sacred trees

I spent many months in the field, along with many other researchers, trying to figure out what these chimps are up to. So far we have two main theories. The behavior could be part of a male display, where the loud bang made when a rock hits a hollow tree adds to the impressive nature of a display. This could be especially likely in areas where there are not many trees with large roots that chimps would normally drum on with their powerful hands and feet. If some trees produce an impressive bang, this could accompany or replace feet drumming in a display, and trees with particularly good acoustics could become popular spots for revisits.

On the other hand, it could be more symbolic than that—and more reminiscent of our own past. Marking pathways and territories with signposts—piles of rocks, for example—is an important step in human history. Figuring out where chimps’ territories are in relation to rock-throwing sites could give us insights into whether this is the case here.

Even more intriguing than this, maybe we found the first evidence of chimpanzees creating a kind of shrine that could indicate sacred trees. Indigenous West African people have stone collections at “sacred” trees; such man-made stone collections are commonly observed across the world, and look eerily similar to what we have discovered here.

images of stone throwing
Stone throwing, in-action and on-site. Top line: Adult male tossing, hurling, and banging a stone. Bottom line: Stones accumulated in a hollow tree; typical stone throwing site; and stones in between large roots.
Image: Kühl et al (2016) | Provided by author

A vanishing world

To unravel the mysteries of our closest living relatives, we must make space for them in the wild. In the Ivory Coast alone, chimpanzee populations have decreased by more than 90% in the past 17 years.

A devastating combination of increasing human numbers, habitat destruction, poaching, and infectious disease severely endangers chimpanzees. Leading scientists warn us that, if nothing changes, chimps and other great apes will have only 30 years left in the wild. In the unprotected forests of Guinea, where we first discovered this enigmatic behavior, rapid deforestation is rendering the area close to uninhabitable for the chimps that once lived and thrived there. Allowing chimpanzees in the wild to continue spiraling toward extinction will not only be a critical loss to biodiversity, but a tragic loss to our own heritage, too.

You can support chimps with your time, by instantly becoming a citizen-scientist and spying on them at www.chimpandsee.org, and with your wallet by donating to the Wild Chimpanzee Foundation. Who knows what we might find next that could forever change our understanding of our closest relatives.

This post originally appeared at The Conversation. Follow @ConversationUS on Twitter.

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