The Influence of Target Animacy and Social Rank on Hand Preference in Barbary Macaques (Macaca sylvanus)

Brain hemispheres have different functions and control the movements of the contralateral side of the body. One of these functions is processing emotions. The right hemisphere hypothesis suggests that the right hemisphere of the brain is responsible for emotional processing, so the left side of the body is activated in emotive contexts such as social interactions. In contrast, the valence hypothesis proposes that both hemispheres are involved in emotional processing, with the left hemisphere processing positive emotions and the right hemisphere negative emotions. We investigated whether and how interaction with inanimate and animate targets affected manual laterality in 12 zoo-housed Barbary macaques (Macaca sylvanus). We focused on the direction and the strength of hand preference and tested the effect of social rank on lateralization. We used continuous focal animal sampling to record bouts of hand preference when interacting with inanimate targets (fourteen 15-min samples) and animate targets (during social and self-directed behaviors, fourteen 2-h morning samples and 14 90- min afternoon samples) and recorded social interactions to measure rank. At the individual level, six of nine lateralized macaques were significantly right-handed when interacting with inanimate targets, whereas only three subjects showed a significant (right) lateralization when interacting with animate targets. Thus, inanimate targets seem to elicit manual laterality to a greater extent than animate targets. At the group level, we found no hand preference for actions directed toward inanimate or animate targets in general but we found a right-hand bias for affiliative behaviors. There was no effect of social rank on lateralization. Despite the limitations of a small sample size, our results suggest that both hemispheres influence hand preference during social interactions, supporting the valence hypothesis.