New study reveals that visible traces of past tool use can outweigh material properties when primates select tools
This capuchin monkey cracks open a cashew nut using the experimental stones. It achieves this by placing the nut on a soft anvil and striking it with a hard hammerstone.
© Tiago Falótico
To the point
- Tool selection strategies: Wild bearded capuchin monkeys match the hardness of stones to their intended function. When given a free choice, they prefer to use softer stones as anvils and harder stones as hammers.
- Trusting the evidence: Monkeys prefer "used" stones that show visible signs of previous successful nut-cracking over raw ones, even when the material properties are less than ideal.
- Indirect social learning: The findings suggest that capuchins can learn from the lasting environmental traces left by others without ever having observed the event of tool use itself.
- Implications for human origins: The study provides a fresh perspective on interpreting early hominin archaeological sites, where the accumulation of used stones may have attracted repeated visits and reuse.
A new study provides experimental evidence that wild capuchin monkeys use more than just physical cues when selecting stone tools; they also rely on visible signs that a stone has previously been used for this purpose. Researchers from the Max Planck Institute for Evolutionary Anthropology and the University of São Paulo investigated how nut-cracking capuchin monkeys choose between stones of different hardness and appearance, examining whether evidence of prior use, as indicated by percussive damage, can override the influence of a stone's intrinsic material properties.
The team studied two wild populations of bearded capuchin monkeys (Sapajus libidinosus) in Brazil and presented them with standardised stones in three experimental tasks. For each task, the monkeys could choose between raw, unmodified stones and stones bearing visible signs of previous nut-cracking activity.
Matching materials to the task
In the first task, when all the stones were unused, the monkeys consistently selected different materials for different purposes: softer stones for anvils and harder stones for hammerstones. This suggests that capuchins have an understanding of how material properties relate to mechanical function.
"What surprised us was how consistently the monkeys distinguished between stone types based purely on hardness, with no prior visible use," says Johanna Henke-von der Malsburg, the study's first author and a former postdoctoral researcher at the Max Planck Institute for Evolutionary Anthropology who is now a postdoctoral researcher at the Research Institute for Farm Animal Biology. "This suggests that functional stone selection is not just a habit; it demonstrates genuine awareness of the physical requirements of the task."
When evidence of use takes over
In the second experiment, the monkeys were given a choice between an entirely unused nut-cracking site and one that already contained battered stones and broken nutshells. The monkeys showed a strong preference for the previously used site and, within it, continued to favour softer anvils and harder hammer stones, as well as stones that showed visible wear.
The third task pitted raw and used stones against each other directly. Here, the pattern shifted: for anvils, signs of previous use were favoured over material hardness, with monkeys preferring used stones even when they were not the mechanically optimal choice. However, no clear preference for hardness emerged for hammerstones once visible use entered the equation.
"This is a striking case where social information appears to override an individual's assessment of physical properties," explains Lydia Lunz, the study's senior author and a research group leader at the Max Planck Institute for Evolutionary Anthropology. "The monkeys didn't need to see another individual crack a nut. The traces left behind - the damage to the stone - were enough to guide their decision. This is a subtle yet powerful form of social learning."
Learning from traces, not just demonstrations
The three experiments together demonstrate that social learning in wild capuchins does not always require the direct observation of another individual's behaviour. Instead, durable physical traces, such as battered stones and discarded nutshells, can transmit information about how and where a task has been successfully completed in the past.
This form of indirect social information may play a significant yet often overlooked role in transmitting and maintaining tool-use traditions across generations of primates. This allows behaviours to persist and spread even in the absence of direct teaching or imitation.
Rethinking the archaeological record
The findings may also provide archaeologists studying early hominin sites with new interpretive tools. Accumulations of modified stones at particular locations have traditionally been interpreted primarily in terms of the availability of raw materials or the practical demands of the task at hand. However, this study suggests an additional factor: the mere presence of visibly modified stones that had previously been used may have actively drawn early hominins back to specific places, encouraging the reuse of both the location and its available materials.
"If modern wild primates can be guided by the accumulated evidence of past tool use, this raises important questions about how early hominins may have related to their discarded tools and worked stones," says Lunz. "The archaeological landscape itself may have functioned as a kind of external memory, influencing where and how tool traditions emerged and persisted."
