Chimpanzees Throwing Rocks at Trees: A World First

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 savannahs of the Republic of Guinea, I stumbled awkwardly through thick undergrowth, unsuccessfully trying to last five minutes without becoming caught in thorns that seemed to obstruct every step. Our purpose was to document and learn about a community of wild chimpanzees that had never previously been researched. Rather than living with the security of a protected area, these chimpanzees survive in fragments of forest scattered among farms and villages.

We stopped in a clearing in the bush. Relieved to be beyond the reach of thorns, I nevertheless wondered why we had halted. I moved to the front of our party to ask Mamadou Alioh Bah, the village chief and our renowned guide. He said he had noticed something intriguing: seemingly unremarkable marks on a tree trunk.

In the busy, tangled setting of the savannah, it was something most of us would not have spotted, yet it had made him stop. Among our group of six, some thought wild pigs had left the marks while rubbing against the trunk, while others suspected teenagers playing about.

Alioh, however, had a feeling. When someone capable of locating a single shed chimp hair on the forest floor, and of seeing chimpanzees kilometres away with the naked eye more effectively than you can with costly binoculars, has a feeling, you pay attention. We installed a camera trap, hoping that whatever had made the marks would return and repeat the behaviour - this time on film.

A world first

Camera traps begin filming automatically whenever something moves in front of them. That makes them an excellent way to record wildlife behaving naturally without disturbance. I noted that we should come back to the same location in two weeks - approximately the lifespan of the batteries - and we continued into the wilderness.

Returning to a camera trap always brings an air of excitement about what mysteries it might contain. Most of our footage, admittedly, showed branches moving in high winds or farmers’ cows enthusiastically licking the lens, but there is always an irresistible expectation that it may have captured something extraordinary.

The footage from this camera was thrilling. A large male chimpanzee walked towards our mystery tree and stopped briefly. He looked around quickly, picked up a huge rock, and hurled it with full force against the trunk.

We had never observed anything like it, and it gave me goosebumps. Jane Goodall first identified wild chimpanzees using tools in the 1960s. Chimpanzees use twigs, leaves and sticks, and some communities even use spears, to obtain food. They also use stones to crack nuts and open large fruits. On occasion, chimpanzees throw rocks as displays of strength to establish their standing within a community.

However, our now-published study revealed not an isolated, chance occurrence but a repeated practice with no obvious connection to acquiring food or status - perhaps it is a ritual. After searching the surrounding area, we identified many further locations where trees bore comparable marks. At numerous sites, rocks had built up inside hollow trunks, recalling the stone piles uncovered by archaeologists from human history.

Footage soon began arriving. Other teams involved in our project started looking for trees bearing the revealing marks. We recorded the same puzzling behaviour in small areas of Guinea-Bissau, Liberia and Côte d’Ivoire, but nowhere farther east, despite surveys throughout the chimpanzee range, from Guinea’s western 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 activity may form part of a male display, with the loud impact of a rock striking a hollow tree increasing the display’s impressiveness.

This may be particularly plausible where few trees have the large roots that chimpanzees would normally drum using their powerful hands and feet. If particular trees create a striking bang, the sound could complement - or take the place of - foot drumming during a display, and trees with especially effective acoustics may become favoured places to revisit.

Alternatively, the practice could carry a more symbolic meaning, and have stronger parallels with our own history. Using markers such as rock piles to indicate routes and territories was an important development in human history. Establishing how chimpanzee territories relate to the sites where rocks are thrown could help us determine whether that is happening here.

More intriguingly still, perhaps we have found the first indication that chimpanzees create a form of shrine, suggesting the existence of sacred trees. Indigenous West African peoples maintain stone collections at “sacred” trees, and these human-made collections are widely seen around the world. They look strikingly similar to the discoveries made here.

A vanishing world

To solve the mysteries surrounding our nearest 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 face grave danger from the devastating combination of growing human populations, habitat loss, poaching and infectious disease. Prominent scientists have warned 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 documented this enigmatic behaviour, swift deforestation is making the landscape nearly impossible to inhabit for the chimpanzees that once lived and flourished there. Letting wild chimpanzees continue their slide towards extinction would represent not only a severe loss of 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 support them financially by donating to the Wild Chimpanzee Foundation. Who knows what we may discover next that could permanently transform our understanding of our closest 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.