Chimpanzees throw rocks at trees in a possible ritual

This article was written by Laura Kehoe of Humboldt University of Berlin and was first published by The Conversation.

On my first field expedition in the savannah of the Republic of Guinea, I stumbled awkwardly through thick undergrowth, unsuccessfully trying to last even five minutes without becoming caught in thorns that seemed to snag me at every turn. Our purpose was to observe and learn about a community of wild chimpanzees that had never previously been studied. Rather than living within the relative security of a protected area, these chimpanzees survive in fragments of forest lying between farms and villages.

We stopped in a clearing in the bush. Relieved to be out of reach of the thorns, I wondered why we had halted. I went to the front of our party to ask Mamadou Alioh Bah, the village chief and our renowned guide. He said he had spotted something unusual: apparently unremarkable marks on a tree trunk.

In the intricate, cluttered setting of the savannah, it was something most of us would never have noticed, yet it had made him stop. Among our group of six, some thought the marks had been left by wild pigs rubbing against the trunk, while others suspected teenagers playing about.

Alioh, however, had a feeling about it. When someone who can locate one fallen chimpanzee hair on the forest floor, and spot chimpanzees kilometres away with the naked eye more effectively than you can with costly binoculars, has a hunch, it is worth listening. We installed a camera trap, hoping that whatever had made the marks would return and repeat the action - this time on film.

A world first

Camera traps begin filming automatically when movement occurs in front of them, making them an excellent way to record wildlife behaving naturally without disturbance. I noted that we should come back to the location in two weeks, roughly the lifespan of the batteries, and we continued into the wilderness.

Returning to a camera trap always brings a feeling of excitement about what secrets it may contain. Most of our footage showed branches moving in high winds or farmers’ cows wandering past and eagerly licking the lens, but there is still an irresistible sense that the camera might have captured something extraordinary.

The recording from this camera was electrifying. A large male chimpanzee walked up to our mystery tree and stopped briefly. He looked around quickly, picked up an enormous rock, then hurled it at the trunk with all his strength.

It gave me goosebumps, because nothing like it had ever been documented. Jane Goodall first identified wild chimpanzees using tools during the 1960s. Chimpanzees employ twigs, leaves and sticks - and some communities even use spears - to obtain food. They also use stones to crack nuts and open large fruit. On occasion, chimpanzees throw rocks as displays of power intended to establish their standing within a community.

However, our now-published study showed that this was not an isolated, chance occurrence. It was a repeated behaviour with no obvious connection to obtaining food or status, and it may be a ritual. We investigated the surrounding area and identified numerous further sites with similarly marked trees. At many of them, rocks had built up inside hollow trunks, recalling the stone piles archaeologists have found throughout human history.

More videos followed. Other teams working on our project began looking for trees bearing the distinctive signs. We recorded the same puzzling activity in small areas of Guinea-Bissau, Liberia and Côte d’Ivoire, but not farther east, despite searches throughout the chimpanzee range from Guinea’s west coast to Tanzania.

Sacred trees

Together with many other researchers, I spent months in the field attempting to understand what these chimpanzees were doing. At present, we have two principal explanations. The behaviour may form part of a male display, with the loud crash of a stone striking a hollow tree making that display still more impressive.

This may be particularly plausible where there are few trees with large roots, which chimpanzees usually drum with their powerful hands and feet. Where particular trees create an impressive sound, the impact could supplement or replace foot drumming during a display. Trees with especially effective acoustics might consequently become favoured places to revisit.

Alternatively, the action could have a more symbolic meaning, one that is closer to our own history. Using signposts, including piles of stones, to mark routes and territories represented an important stage in human history. Establishing how chimpanzee territories relate to sites where rocks are thrown could help us determine whether this explanation applies.

More intriguingly still, we may have uncovered the first evidence of chimpanzees making a type of shrine, potentially pointing to sacred trees. Indigenous people in West Africa maintain stone collections at “sacred” trees. Such human-made stone collections are widely seen around the world, and they look strikingly similar to those we found.

A vanishing world

To solve the mysteries surrounding our closest living relatives, we need to leave room for them in the wild. In Côte d’Ivoire alone, chimpanzee populations have fallen by more than 90 percent over the past 17 years.

Chimpanzees are gravely threatened by the destructive combination of growing human populations, habitat loss, poaching and infectious disease. Leading scientists warn that, unless circumstances change, chimpanzees and other great apes may have only 30 years remaining in the wild.

In Guinea’s unprotected forests, where we initially observed this enigmatic behaviour, rapid deforestation is making the landscape almost unliveable for chimpanzees that once inhabited and flourished there. Letting wild chimpanzees continue their descent towards extinction would be not only a severe blow to biodiversity, but also a tragic loss of our own heritage.

You can help chimpanzees by giving your time: become a citizen scientist immediately and watch them at chimpandsee.org. You can also contribute financially by donating to the Wild Chimpanzee Foundation. Who knows what we may discover next that could permanently transform our understanding of our nearest relatives.

Laura Kehoe, PhD researcher in wildlife conservation and land use, Humboldt University of Berlin

This article was originally published by The Conversation. Read the original article.