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

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.