In the savannah of Guinea and the forests of Liberia the chimpanzees have an unusual hobby. They like to pick up stones and throw them at a tree. Or sometimes they just place their stones gently at the base, creating a cairn.
The primatologists who discovered this behaviour say that it is possible that the practice, which seems to serve no practical purpose, may be evidence of ritualistic behaviour in non-humans.
Laura Kehoe was walking through the thick savannah of the Republic of Guinea studying wild chimpanzees when one of her colleagues spotted odd scuff marks in the bark of a tree. They thought they may have been caused by previously unseen chimpanzee behaviour and decided to investigate.
The chimpanzees were not accustomed to humans, though, and because of the threat of poachers the researchers did not want this to change, so they set up a camera to spy on them. When they came back and reviewed the footage days later they were amazed to see chimpanzees coming the tree, picking up stones and flinging them at its base.
What is more, after they alerted other researchers they received reports of similar behaviour across West Africa but not in eastern populations. “It’s very intriguing,” said Ms Kehoe, who is studying for a PhD at the Humboldt University of Berlin. “I’ve spent so much time thinking about this and what it could mean.”
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In a paper published in the journal Nature Scientific Reports she and her colleagues suggested two explanations. “Maybe it is part of a display ritual, a way of communicating long distance,” she said. Male chimpanzees drum on tree roots to assert their presence. “Maybe in this habitat there are not so many large trees for drumming, maybe throwing a rock makes a louder sound.”
The problem with this theory is that in some videos the behaviour was very different. On one occasion a juvenile chimp quietly placed stones inside the tree. “That poses questions,” she said. Could it be that instead the tree has some symbolic, ritualistic significance?
Such cairns are evident in human societies all round the world. “Marking pathways and territories with signposts such as piles of rocks is an important step in human history,” said Ms Kehoe, writing about the discovery on the academic website The Conversation. “Maybe we found the first evidence of chimpanzees creating a kind of shrine that could indicate sacred trees.”
In West Africa some indigenous people collect stones into piles at trees that they consider sacred. Is it conceivable this is a similar practice? If so, it would imply that symbolic, even religious, behaviours are not confined to humans.
Finding out what is going on will be difficult, partly because the chimpanzees are shy but also because they are endangered. “The areas where chimps are doing this are becoming rapidly deforested. We need to make sure that the chimps in the wild survive, otherwise we will never figure out what they are doing,” Ms Kehoe said.