Task complexity, posture, age, sex: which is the main factor influencing manual laterality in captive Cercocebus torquatus torquatus?

Behavioural asymmetries reflect brain asymmetry in nonhuman primates (NHP) as in humans. By investigating manual laterality, researchers can study the evolution of brain hemisphere specialisation. Three dominant theories aim to establish an evolutionary scenario. The most recent theory relates different levels of manual laterality to task complexity. Our investigation aimed to evaluate the importance of two extrinsic factors (posture and the need for manual coordination) and two intrinsic factors (age and sex) on the expression of manual laterality by red-capped mangabeys. We observed 19 captive-born mangabeys, in spontaneous situations and under experimental conditions (seven experimental tasks varying in complexity). No directionality was observed in hand preference at the group level whatever the task. But our data revealed an effect of task complexity: more subjects were lateralised than not lateralised for the bipedal task and for the three most complex tasks. Finally, we evidenced an age and a sex effect. We compare our results with data for several other primate species and discuss them in the light of different manual laterality theories.