Ritualized behavior? Chimps all throw rocks at the same tree

(credit: Mark Linfield/Walt Disney Pictures, CC BY)

Camera trap footage, taken with no humans present to cause a disturbance, shows one chimp after another pick up a rock and hurl it at the same tree. Rocks pile up at the foot of the tree, which starts to show signs of wear and tear. For some reason, the chimps have picked this particular tree for an accumulation of hurled rocks.

"It was unlike anything I had ever observed among wild chimpanzees," said primatologist Ammie Kalan. Her team has discovered the behavior in four distinct populations, and it's possible that more will turn up as they continue searching. What the rock piles mean is an open question, but the discovery of such a distinct and puzzling stone tool use is unquestionably exciting.

We've known for a while now that chimps use stone tools for more easily discernible purposes, like extracting food. They've also been observed throwing objects, like rocks and branches, haphazardly. What's different about this behavior is the consistent location and the repetitive actions seen in multiple individuals, which is what has led the researchers to label it as tool use. "It's definitely the first indication of something else going on other than stone tool use for food extraction or for throwing rocks" haphazardly," Kalan told Ars.

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