Target animacy influences chimpanzee handedness

Abstract

We employed a bottom-up, quantitative method to investigate great ape handedness. Our previous investigation of gorillas (Gorilla gorilla gorilla) demonstrated that contextual information influenced an individual’s handedness toward target objects. Specifically, we found a significant right-hand bias for unimanual actions directed toward inanimate target objects but not for actions directed to animate target objects (Forrester et al. in Anim Cogn 14(6):903–907, 2011). Using the identical methodological technique, we investigated the spontaneous hand actions of nine captive chimpanzees (Pan troglodytes) during naturalistic, spontaneous behavior. We assessed both the frequencies and proportions of lateralized hand actions directed toward animate and inanimate targets employing focal follow video sampling. Like the gorillas, the chimpanzees demonstrated a right-handed bias for actions directed toward inanimate targets, but not toward animate targets. This pattern was evident at the group level and for the majority of subjects at the individual level. We postulate that a right-hand bias for only inanimate targets reflects the left hemisphere’s dominant neural processing capabilities for objects that have functional properties (inanimate objects). We further speculate that a population-level right-hand bias is not a human-unique characteristic, but one that was inherited from a common human-ape ancestor.