Sticks and Stones: Hand Preference and Grip in Wild Chimpanzee Tool-Use.

OBJECTIVES

Humans exhibit a near-universal right-handed bias, linked to hemispheric brain lateralization, yet its evolutionary origins remain unclear. Wild chimpanzees, one of our closest living relatives, show diverse tool-use behaviors and substantial variability, providing an appropriate context to investigate the emergence and functional significance of handedness in ecologically relevant settings. This study aimed to assess chimpanzee handedness and grip at both the individual and behavioral levels to determine whether specific tool-use tasks elicit consistent preferences.

METHODS

We investigated hand preferences and grasping patterns in wild chimpanzees for three tool-use behaviors observed in three different communities: accumulative stone throwing, algae fishing, and nut cracking. Our dataset consisted of camera-trap videos of unhabituated wild chimpanzees with individuals identified across videos.

RESULTS

We found 20/21 individual chimpanzees were consistent in their hand preference during primary tool-use actions: fishing, hammering, or throwing. Insufficient observations of a further 16 chimpanzees prevented assessment of their hand preference. We found no evidence for behavior- or population-level biases in hand preference. Grips were only possible to code for algae fishing, where we found pronounced inter-individual variability in grasping, with notable trends between left- and right-handed chimpanzees. Also, right-handed individuals appeared to fish for longer durations, although the implications for tool-use performance remain unclear.

DISCUSSION: This study highlights challenges and complexity associated with plant-based precision-grasping tool-use behaviors like algae fishing. During the early stages of hominin evolution, the efficient execution of such manual behaviors likely benefited from key evolutionary adaptations favoring improved dexterity, motor skills, and enhanced brain-hand coordination.