How does object play shape tool use emergence? Integrating observations and field experiments in longtailed macaques

It has long been suggested that object play facilitates the development and evolution of tool use, through enhanced perception of an object's properties and potential for manipulation. However, ecologically relevant support for this claim is scant. We examined whether a form of culturally maintained object play, named stone handling, characterized by high interindividual variation in its behavioural expression, promotes the acquisition and further expression of stone-tool use in a nonhuman primate species.

We conducted a series of field experiments in a free-ranging group of Balinese longtailed macaques, Macaca fascicularis, to test whether the stone-handling profiles of different individuals predicted their ability to solve a foraging task, whose solution required the functional and action-specific use of stones as tools. Frequentist network-based diffusion analysis, Bayesian multilevel regression modelling and descriptions of individuals' learning trajectories showed that the solutions to different foraging tasks required varying reliance on social and asocial learning strategies. Our results suggest that certain stone-handling profiles may increase an individual's likelihood of expressing stone-tool use. However, other trait- and state-dependent variables may also contribute to explaining individual differences in the development and expression of stone-tool use. The behavioural idiosyncrasies associated with stone handling in longtailed macaques may serve as an exaptive reservoir for the possible emergence of stone-tool use. To our knowledge, this is the first study to experimentally evaluate the role of stone-directed play in the acquisition of stone-tool use.