The development of expertise at cracking palm nuts by wild bearded capuchin monkeys, Sapajus libidinosus.

Bearded capuchin monkeys at Fazenda Boa Vista, Piauí, Brazil older than 8 years routinely crack palm nuts with a stone hammer and anvil. An embodied, perception–action stance hypothesizes that monkeys' improving skill at this task is enabled by learning affordances of spatial and force relations produced with objects. Affordances are individual-relative opportunities for action that may change across development. We evaluated predictions linking behaviours with nuts and stones and body mass to success at nut cracking in young bearded capuchins (1.3–6.3 years old) and adults over three annual observation periods. Behaviours with nuts and stones reflected the monkeys' learning affordances for cracking; body mass constrained affordances for cracking. For each period in which they were observed, individuals were assigned to novice, intermediate or expert classes in accord with their success at cracking nuts. Novices did not crack nuts, despite striking them with stones. Body mass was modestly associated with intermediates', but not experts', likelihood of cracking a nut. Experts exhibited higher proportions of manipulation devoted to positioning nuts on the anvil and shorter bouts of cracking than intermediates; intermediates exhibited greater variability in these variables than experts or novices. The strongest classification model predicting individuals' assignments to skill class with 89% accuracy combined average bout duration and the ratio of manipulation devoted to positioning to other manipulation. The findings suggest practise using the body-pus-tool system for cracking palm nuts supports affordance learning and results in gradual mastery of this skill and that changing body mass plays a small role in this process. • Most monkeys <7 years old did not have adult (expert) skill at cracking nuts. • Body mass had little to no effect on monkeys' skill at cracking nuts. • Experts cracked nuts in short bouts and relatively frequently positioned the nut. • Less skilled monkeys used long bouts and varied duration and action more over bouts. • Monkeys slowly learn affordances of the body-plus-tool system for nut cracking. [ABSTRACT FROM AUTHOR]