Researchers studying dozens of wild chimpanzee communities have recorded many of the apes banging and throwing rocks against trees, creating conspicuous accumulations of stones. The findings, published in Scientific Reports this week, describe the first repeated observations of individual chimps using stone tools for a purpose other than extractive foraging.

Second to humans, chimpanzees (Pan troglodytes) display the greatest variation in tool-use behaviors. They use leaves, sticks, and twigs to acquire food, stone hammers and anvils to crack open nuts, and stone cleavers to cut up large fruits. But what we know about their tool-use repertoire is limited to a patchy distribution of long-term field sites.

To help overcome this limitation, a huge international team led by Ammie Kalan from the Max Planck Institute for Evolutionary Anthropology studied a total of 34 chimpanzee communities across the species’ range as part of the Pan African Programme: The Cultured Chimpanzee (PanAf). Data collection at each PanAf site is conducted for periods lasting 14 to 17 months. The team also examined published data from 17 mid- to long-term chimpanzee research sites. 

Field workers first heard stone banging in 2010 at Boé, Guinea-Bissau, and the first confirmed observation of accumulative stone throwing occurred in 2011 at Sangaredi, Guinea, after peculiar markings were discovered on a hollow tree. The team then strapped remote video camera traps on trees near locations where signs of stone throwing or piling behavior were found. When reviewing the videos, they classified stone-throwing behavior based on the type of handling observed: bang, hurl, or toss. Hurling means they used one or both hands to throw a rock at the tree, while tossing means they threw the rock into a hollowed-out tree or a hollow groove formed by buttress roots.

The researchers discovered that chimps from four populations in West Africa – Guinea-Bissau, Guinea, Liberia, and Côte d’Ivoire – habitually banged and threw rocks against trees or tossed them into tree cavities. These behaviors resulted in conspicuous signs of wear and stone accumulations containing up to 37 rocks. The team recorded a total of 64 stone throwing events: Either the chimps held a stone in their hands or they had grabbed one, then threw it. 

These throwers were mostly adult males, and in almost all of the instances, stone throwing was accompanied by what’s called the “pant hoot,” part of a ritualized aggressive display that also involves drumming with the hands and feet. It’s possible that the banging could be a modification of the male display that enhances sound propagation. 

Or perhaps the stone piles help mark territory boundaries. Researchers studying the remains of our extinct ancestors often rely on comparisons with living primates to help shed light on prehistoric behaviors. Stone throwing and the accumulation that results are reminiscent of human cairns. Ancient and modern human societies used stone piles to mark food caches or pathways. Some of these sites might also hold a more symbolic meaning for burials or to help establish shrines.