Stone tool use by wild capuchin monkeys: a new reference point for investigating tool use across species and evolutionary time

Tool-use and technology are essential to the survival of human species. In recent decades the study of non-human animals tool-using behaviours has changed the way we think about the role of tools in the natural world. This developing dataset, gathered across multiple species and from multiple perspectives, is the key to understanding the adaptive benefits of tool-use in its wider evolutionary context. We recently discovered that wild bearded capuchin monkeys (Sapajus libidinosus), a species that separated from the human lineage about 35 million years ago, use hammers and anvils to crack open nuts and access their nutritious kernels. Both nut cracking and tool transport require bipedal stance, reflect considerable motor skills, and have high energetic costs. Nut cracking activities modify the surface of the anvil sites and the same anvils are repeatedly used since after use tools are left there, or nearby. Observational and experimental field studies on the cracking activities of two groups of capuchin monkeys living in a Brazilian semi-arid forest habitat show that monkeys perform tool use throughout the year and use proportionally large stones (weighing on average, 1 kg) in relation to their body mass (an adult female weighs about 2 kg, a male up to 4.5 kg). We found that the nuts are very difficult to crack because of their thick, tough shell and that stones suitable as hammers are very rare in the habitat where capuchins live. Furthermore, several experiments demonstrated that capuchins are very selective in their choice of stones, nuts, and anvil sites. Our findings challenge notions that selectivity, transport and physical skills in tool use are characteristic only of human beings, human ancestors, and great apes.