Influences and constraints on manual asymmetry in wild African primates: Reassessing implications for the evolution of human handedness and brain lateralization
The association between right-handedness and left hemispheric brain specialization for speech and language is well-documented in humans, yet the proximate “causes” of handedness and brain asymmetry remain to be elucidated. Researchers interested in the evolution of the human brain and language have used assessments of handedness to infer brain lateralization, both in living species and in analysis of prehistoric material. Non-human handedness studies have historically focused on testing artificial tasks in laboratory animals, yielding inconsistent results. In contrast, this study is a cross-species investigation of hand usage in wild, non-human primates. Fieldwork identified and measured variables capable of influencing and/or constraining hand preferences of the following species in their natural context: common chimpanzee (Pan troglodytes), red colobus (Colobus badius), redtail monkey (Cercopithecus ascanius), grey-cheeked mangabey (Cerocebus albigena) at Kibale Forest, Uganda, and mountain gorilla (Gorilla g. beringei ) at Pare National des Volcans, Rwanda.
No species-level left- or right-handedness was found for any of the five species studied, either overall, by task, by sex, or by age. Rather, study groups tended to be composed of equal numbers of left- and right-handed individuals; monkeys and apes demonstrated individual directional hand preferences that were mostly consistent across tasks, but sometimes varied. However, assessments of the relative strength of hand preference (regardless of direction) revealed manual behavior patterns related to age, task, and relative postural stability. Results strongly upheld the prediction that animals will express less manual asymmetry under conditions of postural instability/biomechanical constraint.
Handedness research is limited by serious problems. No standard, empirically-based measure of handedness exists; indices of handedness are arbitrary, causing findings to vary significantly. For this and other reasons (neurological, taphonomic), current assessments of handedness are not sufficient to infer brain lateralization in hominids. Even in extant species, consistent manual asymmetry does not necessarily imply brain lateralization. I propose a new approach to address two fundamental issues: (1) identification of empirically-based criteria for categories and measures of handedness and (2) better understanding of the relationship between handedness and functional hemispheric specializations.