We investigated manual coordination and manual preference in stone manipulation. Comprised of multiple one-handed and (a)symmetrical/(un)coordinated two-handed patterns, stone handling is a good candidate for the study of complexity and laterality in object manipulation by Japanese macaques. We presented a cross-sectional developmental analysis of stone handling complexity, through the combined investigation of bimanuality, coordination, and symmetry in hand use. [REPORT]

For individual subjects, we found evidence for manual lateral bias within particular stone handling patterns (hand preference) and consistency in laterality across stone handling patterns (hand specialization). At the group-level, we found that, although their collective distribution of left versus right remained random, most subjects were either significantly but incompletely lateralized, or completely lateralized within particular stone handling patterns (pattern specialization), but not across all stone handling patterns (no handedness for stone handling behavior as a whole). As predicted by the task-complexity model, hand specialization and handedness were stronger in the coordinated bimanual stone handling patterns than in the unimanual patterns. [REPORT]

Our findings have implications for the evolution of manual coordination and laterality in non-instrumental object manipulation versus stone-tool use in hominids.