Wild Chimpanzees Have Been Throwing Rocks at the Same Trees for Over a Decade

In Boé National Park in Guinea-Bissau, some trees stand covered in gnarled scars with piles of rocks heaped at their base — evidence of a rare chimpanzee behaviour called accumulative stone throwing.

Video recordings show wild western chimpanzees, usually adult males, repeatedly hurling rocks at the same trees. As they throw, the chimpanzees pant hoot, a loud call used for long-distance communication, and sometimes drum on the tree's buttress roots with their hands and feet.

Researchers travelling to the remote Boé chimpanzee territory used camera traps and audio recorders to study the behaviour, since the local chimpanzees are unhabituated and flee from humans. The team found that most of the stone-throwing sites first identified by the Pan African Programme, and revisited in 2017, were still in use during their latest trip — meaning chimpanzees have returned to the same trees for more than a decade.

Researchers say the behaviour is likely cultural rather than simply a product of where rocks and trees happen to be available, since sites with both do not always become stone-throwing spots. It has been recorded in only four chimpanzee groups in West Africa so far, and scientists still do not know exactly what the chimpanzees are trying to communicate, though earlier research suggests the sites may mark important locations in the chimpanzees' territory.

While in the field, the researchers found bore holes left by bauxite mining exploration in the chimpanzees' habitat in Guinea-Bissau, an industry already linked to habitat destruction and pollution in neighbouring Guinea.