On the proximate links between object play and tool use in the context of stone handling behavior in Balinese long-tailed macaques

Abstract

Several theories on the origins and evolution of instrumental object-assisted actions hold that object play facilitates tool use, through enhanced perception of an object’s properties and potential for manipulation. However, the data-based findings actually connecting these activities are conflicting. In this thesis, I explored the links between object play and tool use at a proximate level, that is by looking at their mechanisms and structural differences. Using a combination of observational and experimental methods, I studied a culturally maintained form of object play named stone handling (SH) performed by Balinese long-tailed macaques. First, I assessed inter-individual variation and intra-individual consistency in the expression of SH behavior, and whether the physical properties of the objects being manipulated (i.e., stone size) affected an individual’s expression of SH activity. Second, I tested whether SH in this population has the exaptive potential to turn into tool use, spontaneously in the sexual domain, as a form of self-directed tool-assisted masturbation, and via experimental induction in the foraging domain, as extractive tool-assisted foraging techniques to open novel food-baited puzzle boxes. Overall, my findings demonstrate that, due to the intrinsic characteristics of play behavior, such as its combinatorial flexibility, SH may be exapted into tool use under certain motivational domains, and qualitative and quantitative features of playful object manipulation, that is the types and duration of different actions, covary with the expression of instrumental object-assisted solutions. Future investigations aiming to explore the relationship between object play and tool use should focus on structural components of these two activities.