Well-worn wins: Wild capuchin monkeys prefer stones with a proven track record of cracking nuts

A recent study reveals that wild capuchin monkeys exhibit sophisticated decision-making when selecting stone tools for nut-cracking. Researchers from the Max Planck Institute for Evolutionary Anthropology and the University of São Paulo observed two populations of bearded capuchin monkeys in Brazil, presenting them with stones of varying hardness and pre-existing signs of use. **The monkeys consistently chose different stone types for specific purposes: softer stones for anvils and harder stones for hammerstones**, even when all stones were unused.

This suggests that capuchins have a nuanced understanding of how material properties correlate with mechanical function.

**In a pivotal experiment, the monkeys were given a choice between unused stones and those already displaying signs of prior nut-cracking activity, such as battered surfaces and broken shell fragments.** Remarkably, the monkeys displayed a strong preference for previously used stones, continuing to favor softer anvils, harder hammerstones, and stones with visible wear.

This preference was not influenced by the material hardness of the stones themselves; instead, the physical traces left by previous use seemed to override the monkeys' assessment of the stones' intrinsic properties.

**Furthermore, when the monkeys were presented with a direct choice between raw and previously used stones**, softer used stones were preferred for anvils even when harder stones would have been mechanically more suitable. In contrast, for hammerstones, no clear preference for hardness emerged once the presence of visible use was considered. This indicates that the monkeys' decision-making process was heavily influenced by social cues conveyed through physical signs of prior use.

**These findings demonstrate that social learning in capuchin monkeys can occur through indirect means**, where durable physical traces such as damaged stones and discarded nutshells relay information about successful tool-use strategies. The presence of these signs may actively draw primates back to specific locations, encouraging the reuse of both the location and its available materials.

This form of social learning, which does not require direct observation of another individual's behavior, highlights the role of durable physical traces in transmitting and maintaining tool-use traditions across generations of primates.

**The study's implications extend beyond capuchin behavior**, potentially offering new insights for archaeologists studying early hominin tool-use sites. Traditionally, accumulations of modified stones have been interpreted as primarily responding to the availability of raw materials or the practical demands of tasks. However, this research suggests that the mere presence of visibly modified stones may have actively influenced early hominins' decision-making, encouraging the reuse of both locations and their associated resources.

This concept challenges the notion that archaeological evidence of tool use should be limited to functional analysis and opens up the possibility that social and environmental factors played a more significant role in shaping early human technological traditions.