Lateralized behavior in white-handed gibbons (Hylobates lar)

Abstract: The evolutionary origins of human handedness are not yet fully understood as evidence of lateralized behavior in nonhuman primates is inconclusive. In the present study, lateralized behavior in both spontaneously occurring motor patterns and a tube task was examined in 15 white-handed gibbons (Hylobates lar). Significant side preferences at the individual level were found within all 15 studied motor patterns. However, no population-level side bias was found for any of the spontaneously occurring or task-related motor patterns and none of the gibbons were consistent in their hand preference across all motor patterns. When only considering the individuals with a significant preference, a significant majority was left-preferent for resting foot. Strength of side preference was significantly higher for the tube task than for all spontaneously occurring motor patterns. Side preferences for manipulation and resting position were significantly stronger than those for supporting hand. Additionally, the preferences for manipulation were significantly stronger than those for leading limb. In the bimanual tube task, females displayed a tendency towards a left-side bias, while males tended to display a bias to the right. Furthermore, females had a significantly stronger hand preference for supporting hand than males. No other sex differences were found. Age, posture, and kinship had no significant effect on lateralized behavior for any of the motor patterns. As in other nonhuman primates, the white-handed gibbons were only consistent in their hand preference across tasks that required similar movements. Altogether, these findings support the notion that population-level handedness may be restricted to human subjects.