Material matters: raw material influences stone tool performance in capuchin monkeys

Identifying the conditions that facilitate tool use is a central focus in the field of human evolution and animal behavior. Particular interest lies in studying stone tool use by non-human primates, especially when stone hammers are used to open encased food sources. Percussive tool use is a rare and complex technological behavior which was also employed by our early hominin ancestors. It is widely theorized that environmental factors play a role in explaining the presence and absence of stone tool use across primate populations. Research to date has focused heavily on the quality and availability of tool-extractable foods, and how these compare to alternative food sources. There has been limited investigation into whether access to different types of raw tool material for hammerstones and anvils affects the efficiency of tool users when exploiting encased resources. Here, we quantitatively analyzed how percussive tools of different raw materials vary in their performance and durability. Wild capuchins (Sapajus libidinosus) in Brazil were provided with stones sourced globally from primate and hominin tool use sites. We measured the reliability with which monkeys could crack nuts when using different raw materials, the number of strikes required to open a nut or obtain a food reward, and how these metrics changed over time. We further reported variations in the durability of different raw materials which directly relates to how long a tool remains usable. Our results showed that differences in nut cracking reliability and efficiency were largely driven by the ability of the tool material to stabilize the nut. Furthermore, there was wide variation in tool durability, particularly for anvils. These results highlight the importance of considering raw materials among the ecological factors that can influence the selective benefits or costs of tool use behaviors. HIGHLIGHTSO_LIRaw material influences primate percussive stone tool durability and performance C_LIO_LIMaterials differ in their ability to stabilize food sources C_LIO_LIEfficiency of hammer strikes differed across materials C_LIO_LIDifferences in material performance did not depend on their capacity to from pits C_LIO_LIMaterial performance has consequences for food yield and energetic efficiency C_LI