The use of stone tools is not a skill unique to humans and our extinct ancestors. Some monkeys also select suitable stones and use them to obtain food that they cannot easily open with their teeth or hands.
Among them are bearded capuchins (Sapajus libidinosus), South American monkeys that place a nut on a flat stone that serves as an anvil and then strike it with another stone used as a hammer to break its hard shell.
New research shows that they do not choose stones at random. According to a press release by the Max Planck Institute for Evolutionary Anthropology, the monkeys recognize the physical properties of different stones, but are particularly attracted to those that already bear traces of successful use.
The research was conducted by scientists from the Max Planck Institute for Evolutionary Anthropology in Leipzig and the University of São Paulo. They studied two populations in Brazil: a wild group in Serra da Capivara National Park, in the state of Piauí, and a semi-free-ranging group in Tietê Ecological Park in São Paulo.
The animals were offered standardized stones of different hardness. When all the stones were new and showed no signs of previous use, the capuchins more often selected softer stones as anvils and harder ones as hammers. This indicates that they can associate particular material properties with the task for which a stone is to be used.
“Stone selection is not just a habit,” says the study’s first author, Johanna Henke-von der Malsburg, who worked at the Max Planck Institute during the research and is now a researcher at Germany’s Research Institute for Farm Animal Biology in Dummerstorf.
In subsequent experiments, the monkeys could choose between a new nut-cracking site and a site where damaged stones and broken shells were already present. They clearly preferred the previously used site.
When the scientists directly offered them new and previously used stones, traces of earlier use sometimes proved more important even than hardness. Particularly when choosing an anvil, the monkeys selected a stone with depressions and signs of wear, even when another stone might have been mechanically better suited to the task.
According to Lydia Luncz, head of the “Technological Primates” research group at the Max Planck Institute and senior author of the study, the monkeys did not need to watch another member of the group cracking nuts. “The traces left on the stone were enough to guide their decision,” she explains.
Scientists describe this as indirect social learning. The animal does not learn through direct observation or teaching, but through changes that others have left in the environment. A worn stone, a depression in an anvil or an accumulation of broken shells can indicate that food had previously been successfully obtained at that location and with that particular tool.
The results were published on 5 August 2026 in the peer-reviewed scientific journal Proceedings of the Royal Society B, in the paper “Social cues on stone tools outweigh raw material properties in wild primates”. The authors include Henke-von der Malsburg, Luncz and researchers from Germany and Brazil.
The discovery could also help archaeologists studying the stone tools of early human ancestors. Accumulations of previously used stones may not simply have been debris from earlier activities, but could also have functioned as a kind of “external memory,” attracting subsequent individuals to return to the same location and use the tools left behind.
