Social laterality and ‘transversality’ in two species of mangabeys: Influence of rank and implication for hemispheric specialization

Individuals in social groups monitor many relationships by adapting their activities to the sex, age, social status, behaviour as well as the position of conspecifics. Here, we investigated the influence of the relative positions of the members of two groups of mangabeys on social visual laterality (right/left) and ‘transversality’ (frontal/rear) in two contexts: (1) ‘one-to-one’ interactions and (2) a ‘one-to-many’ context allowing potential observation of all group members. We discuss our data in relation to (1) the influence of rank and (2) theories explaining lateralization of cerebral hemispheres. First, in ‘one-to-one’ situations, members of both groups were approached more frequently from their left than from their right, and red-capped mangabeys approached a group member more frequently from their right than from their left. In ‘one-to-many’ situations, red-capped mangabeys kept more group members in their right than their left frontal visual field. Conversely, the social ‘transversality’ bias was the same in both contexts: the frontal field was favoured. Second, approach side and relative positions differed according to social rank. Mangabeys that were approached more frequently from their left ranked relatively high. The higher an individual ranked, the more it left other group members behind it; on the contrary dominated mangabeys generally remained below other group members. Thus, social structures, as well as relationships within a group, appear to be good candidates to explain social laterality and ‘transversality’.