Social cues on stone tools outweigh raw material properties in wild primates

Tool use is known among many taxa but its frequency in the animal kingdom is rare. The socio-ecological conditions through which tool use is acquired, transmitted and maintained are the subject of extensive study. Understanding these dynamics offers valuable insight into the evolution of tool use in our species. A fundamental requirement of tool use is the ability to select materials suitable for a given task. In extant primates, material selection is influenced by social and ecological factors; their relative roles, however, remain difficult to disentangle. We present an experimental approach to examine the factors influencing tool selection in robust capuchin monkeys. We provided two naturally tool-using populations with nut-cracking tasks that varied in the material properties of stones and visual evidence of previous use. When presented with unused stones, both populations persistently selected tools based on their material properties. When stones showed signs of previous use, they preferably selected those regardless of material properties. These findings suggest that capuchin monkeys can assess raw material properties but prioritize tools that exhibit evidence of previous nut-cracking activities, indicating that tool selection is shaped by indirect social cues and highlighting the importance of culturally transmitted information in the development of technological skill.