Stone-Tool Use in Wild Monkeys: Implications for the Study of the Body-Plus-Tool System

ABSTRACT

Handheld tools transform the actor's body into a body-plus-tool system such that the fit of the actor's body, the tool, and the task demand channel movement in action. In this study, we performed a biomechanical analysis of percussive actions in wild bearded capuchin monkeys, Sapajus libidinosus at Fazenda Boa Vista, Brazil, as they cracked open tucum nuts with anvil-and-hammer tools from the frame of reference of the body-plus-tool system. The ratio of hammer mass to body mass—the “body-scaled hammer mass”—influenced the monkeys' actions with a hammer and their performance in cracking nuts. Both body mass and hammer mass did not independently influence the monkeys' actions with a hammer and their performance in cracking nuts. These findings support the hypothesis that the properties of the body-plus-tool system are not simply the sum of the properties of the body and the tool.

Acknowledgments

We thank the Fonseca de Oliveira family for logistical help and permission to stay and conduct research at Fazenda Boa Vista. We thank Caroline E. Jones for help with consistency coding the video recordings. We also thank two anonymous reviewers for their valuable feedback on an earlier version of this manuscript. The National Council for Scientific and Technological Development (CNPq, No. 002547/2011–2) and the Authorization and Information System of Biodiversity (SisBio, No. 28689–5), Brazil, permitted us to conduct research in Brazil.

Funding

The University of Georgia funded M.M. and D.M.F.