Is There a “Right” Side of Communicating Friendship? Lateralization of Social Interactions in Zoo Barbary macaques (Macaca sylvanus)

Social laterality in non-human primates has started to attract attention in recent years. The positioning of individuals during social interactions could possibly suggest the nature of a relationship and the social ranking of the subjects involved. The subjects of the present study were 12 adult Barbary macaques (Macaca sylvanus) housed in a zoological garden. We carried out fourteen 210-min video-recorded sessions and we used a focal animal sampling method to collect the position of the subjects during different social interactions. Data on the position of each macaque during three types of social interactions were collected (approach, proximity and affiliative contacts). Moreover, we focused on the outcomes of dyadic agonistic encounters to build the hierarchy of the colony. For each social interaction, two conditions were considered: the side preference (being kept on the left or on the right) and the sagittal preference (being kept in front or on the rear). Bouts of preference of different positions were collected for different social interactions (approach, proximity and contacts). No group-level side preferences were found for any social interaction, suggesting that both hemispheres might be complemental and balance each other during intraspecific communication. For the sagittal preference, we found a group-level bias for proximity, with macaques being kept in front rather than on the rear by close conspecifics. This might be due to the need to detect emotions and intentions of conspecifics. Moreover, high-ranking individuals are kept more frontally than on the rear when in proximity with other macaques. More studies are needed to better investigate social laterality, possibly distinguishing more categories of social interaction, and detecting other variables that might influence the positioning preferences.