Chimpanzees have been throwing rocks at the same trees for more than a decade; scientists are trying to solve the mystery

In the remote savanna-woodlands of Guinea-Bissau, scientists are investigating a strange chimpanzee behaviour that has persisted for more than a decade. Wild western chimpanzees repeatedly collect rocks and hurl them at the same trees, creating piles of stones and leaving visible scars on the trunks. Known as accumulative stone throwing, the behaviour is particularly associated with adult males and often occurs alongside loud vocalisations called pant-hoots and rhythmic strikes against tree trunks. Researchers believe these unusual sites could play a role in chimpanzee communication, territorial displays or social behaviour. However, the exact purpose remains unclear. By studying the trees, stones, sounds and chimpanzees involved, scientists hope to determine why these animals repeatedly return to the same locations and what the behaviour means.

Why are chimpanzees throwing rocks at the same trees

The behaviour is remarkably specific. Rather than randomly throwing stones, the chimpanzees repeatedly target particular trees, often returning to the same locations over many years. In a recent report for The Conversation, researchers Robyn Nakano and Ammie Kalan described their expedition deep into Guinea-Bissau’s Boé region to investigate one of the animal kingdom’s most puzzling behaviours and found that the trees can eventually become visibly marked by the repeated impacts, while rocks accumulate around their bases or inside hollows and grooves formed by large buttress roots. Scientists call this accumulative stone throwing, or AST. It is considered rare because simply having rocks and suitable trees in an area does not automatically lead chimpanzees to create these sites. Researchers have recorded the behaviour in only four groups of wild chimpanzees in West Africa, including populations in Guinea-Bissau, Guinea, Liberia and Côte d’Ivoire. Earlier research documented chimpanzees approaching specific trees with stones before hurling, banging or tossing them against or into the tree. The fact that the behaviour is concentrated in particular populations has raised an intriguing possibility: chimpanzees may have developed and passed on a local behavioural tradition.

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The unusual behaviour may be a form of chimpanzee communication

One of the biggest clues comes from what happens while the chimpanzees throw the stones. The animals often pant-hoop, a loud vocalisation used by chimpanzees for long-distance communication. They may also repeatedly strike the tree with their hands and feet, a behaviour known as buttress drumming. Both behaviours are already associated with male chimpanzee displays. This has led researchers to suggest that stone throwing could be an unusual extension or modification of an established communication display. The possibility is particularly fascinating because the behaviour could mean that the trees themselves have significance to the chimpanzees. Previous research has proposed that accumulative stone-throwing locations could communicate information about territory or mark important places. Some researchers have even suggested that the behaviour might have a symbolic component. However, scientists still do not know what the chimpanzees themselves associate with these locations.

Why do male chimpanzees keep returning to the same trees?

The persistence of these sites is one of the most remarkable discoveries. Researchers working in Guinea-Bissau found that many of the stone-throwing locations originally identified by the Pan African Programme and revisited in 2017 were still being used during their latest fieldwork. That means individual sites can remain part of chimpanzee behaviour for more than a decade. The discovery is important because it suggests the behaviour is not simply an isolated action performed by an individual chimpanzee. Instead, the continued use of particular sites could indicate that information about them is maintained within a social group or passed between generations. It also raises questions about whether chimpanzees have their own forms of cultural traditions.

Researchers travelled deep into Guinea-Bissau to study the chimpanzees

Investigating the behaviour was far from straightforward. The researchers travelled to the remote Boé region of Guinea-Bissau and established a field camp after cycling and hiking about 22 kilometres into the savanna-woodland from the village of Béli. The chimpanzees living there are not habituated to humans. Unlike animals in some long-running research populations, they cannot simply be followed on foot because they will flee when people approach. Instead, the scientists relied on technology. They installed two video cameras at each accumulative stone-throwing site, alongside recording equipment designed to capture sounds around the trees. The researchers also measured trees and created 3D scans of stones that had been thrown at them. By combining visual and audio recordings, they hope to determine exactly who performs the behaviour and how other chimpanzees respond.

Camera traps could reveal what the stone throwing means

The recordings are expected to provide information that is difficult to obtain through direct observation. Scientists want to establish the age and sex of the chimpanzees throwing the stones, as well as whether other chimpanzees are present when the behaviour occurs. The presence of an audience could be particularly important. If chimpanzees regularly throw stones when other members of their group are nearby, the behaviour may be more strongly linked to communication or social signalling. If individuals perform it alone, the explanation could be different. Researchers can also analyse the accompanying vocalisations and body movements to determine whether stone throwing consistently occurs alongside pant-hoots or buttress drumming. Together, those observations could help reveal whether the trees are functioning as physical markers within chimpanzee territories.

Could this behaviour offer clues about human evolution?

Chimpanzees are among humanity’s closest living relatives, which makes unusual behaviours such as accumulative stone throwing particularly valuable to evolutionary researchers. Stone use is already well documented among primates. Some chimpanzees, for example, use stones as tools to access food such as nuts. Accumulative stone throwing is different because it appears to involve stones in a social context rather than simply using them to obtain food. That makes the behaviour potentially significant for understanding how complex communication, symbolic behaviour and material culture may have evolved. If chimpanzees can maintain specific locations and repeatedly use objects at those sites as part of a social display, it could provide researchers with another piece of the puzzle surrounding the evolution of human communication and tool use. The Conversation’s report emphasises that understanding such behaviours may help scientists explore how cultural traditions emerge and persist among great apes.

Guinea-Bissau’s chimpanzees face another growing threat

The mystery of the stone-throwing trees is unfolding against a much more serious conservation problem. Chimpanzee habitat in Guinea-Bissau is increasingly threatened by extractive industries, particularly bauxite mining. During their fieldwork, researchers encountered bore holes associated with mining exploration. Bauxite extraction can bring economic opportunities, but industrial mining can also destroy habitats and cause pollution, creating risks for chimpanzees, other wildlife and nearby communities. For scientists, protecting the chimpanzees therefore means protecting more than individual animals. The stone-throwing sites themselves may represent a form of primate cultural heritage. If the chimpanzees disappear from the region, the behaviour and the locations associated with it could disappear as well. That makes the preservation of their habitat important not only for biodiversity, but also for understanding the cultural lives of one of humanity’s closest living relatives.

Scientists still do not know what the trees mean to chimpanzees

Despite years of observations, there is no definitive answer to why these chimpanzees repeatedly throw rocks at particular trees. The behaviour could be a form of communication, a territorial marker, a social display or perhaps something more symbolic. It may also combine several functions that scientists have not yet identified. What is already clear is that the behaviour is unusually persistent. Trees bearing scars and surrounded by accumulated stones provide physical evidence that chimpanzees have returned to the same locations repeatedly. Some of these sites have remained active for more than ten years, turning an apparently bizarre act of throwing rocks into a potentially important example of animal culture. As researchers analyse the new camera and audio recordings from Guinea-Bissau, the hope is that the footage will finally reveal what happens before, during and after each stone is thrown, and perhaps explain why these chimpanzees keep coming back to the same trees.