How tufted capuchin monkeys (Cebus apella spp) and common chimpanzees (Pan troglodytes) align objects to surfaces: insights into spatial reasoning and implications for tool use.

This report addresses phylogenetic variation in a spatial skill that underlies tool use: aligning objects to a feature of a surface. Fragaszy and Cummins-Sebree's [Behavioral and Cognitive Neuroscience Reviews 4:282-306, 2005] model of relational spatial reasoning and Skill Development and Perception-Action theories guided the design of the study. We examined how capuchins and chimpanzees place stick objects of varying shapes into matching grooves on a flat surface. Although most individuals aligned the long axis of the object with the matching groove more often than expected by chance, all typically did so with poor precision. Some individuals managed to align a second feature, and only one (a capuchin monkey) achieved above-chance success at aligning three features with matching grooves. Our findings suggest that capuchins and chimpanzees do not reliably align objects along even one axis, and that neither species can reliably or easily master object placement tasks that require managing two or more spatial relations concurrently. Moreover, they did not systematically vary their behavior in a manner that would aid discovery of the affordances of the stick-surface combination beyond sliding the stick along the surface (which may have provided haptic information about the location of the groove). These limitations have profound consequences for the forms of tool use we can expect these individuals to master.