After searching West Africa, researchers found sites where chimpanzees had marked trees 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.
Laura Kehoe, who wrote about her experience for The Conservation, was aiming to record and understand a group of wild chimpanzees who had never been studied before. The chimps are not in a protected area, so have to carve out their existence in the patches of forests between farms and villages, she wrote.
With a group, she was tramping through the wilderness in the Republic of Guinea, guided by the chief of the village, Mamadou Alioh Bah, when they stopped in a clearing.
“He told me he had found something interesting – some innocuous markings on a tree trunk. Something that most of us wouldn’t have even noticed in the complex and messy environment of a savannah had stopped him in his tracks.”
“But Alioh had a hunch – and when a man that can find a single fallen chimp hair on the forest floor and can spot chimps kilometres away with his naked eye better than you can (with expensive binoculars) has a hunch, you listen to that hunch. We set up a camera trap in the hope that whatever made these marks would come back and do it again, but this time we would catch it all on film.”
The camera traps automatically start recording when any movement occurs in front of them.
Kehoe returned two weeks later to collect the footage, and the find was “exhilarating” – as she explains, a large male chimp approaches the tree and pauses for a second.
He then quickly glances around, grabs a huge rock and flings it full force at the tree trunk.
“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 she saw on film proved not to be a random, one-off event, instead it was a repeated activity with no clear link to gaining food or status. Kehoe found it could be a ritual.
Other groups working on the project also captured similar activity, finding the same mysterious behaviour 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.
After months researching the activity, Kehoe and other researchers came up with their theories.
Researchers said 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.
Or, as Kehoe writes “it could be more symbolic than that – and more reminiscent of our own past. Marking pathways and territories with signposts such as piles of rocks 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.”
“Indigenous West African people have stone collections at “sacred” trees and such man-made stone collections are commonly observed across the world and look eerily similar to what we have discovered here,” she said.
Through her research Kehoe called for better protection of chimps.
“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 per cent in the past 17 years.”
Increased human populations, habitat destruction, poaching and infectious disease severely endangers chimpanzees, she wrote.
"In the unprotected forests of Guinea, where we first discovered this enigmatic behaviour, 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 spiralling towards extinction will not only be a critical loss to biodiversity, but a tragic loss to our own heritage, too,” Kehoe said.