Is it moving or not? The influence of target animacy and social rank on hand preference in Barbary macaques (Macaca sylvanus)

Abstract

Brain hemispheres have different functions and control the movements of the contralateral side of the body. One of these functions is the process of emotions. The right hemisphere hypothesis suggests that the right hemisphere of the brain is responsible for emotional processing, and 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 dealing with negative emotions. As sociality can influence emotional states, studying behavioral lateralization in social contexts, and the effect of rank on laterality should help to clarify the role of brain hemispheres in social responses. The aim of this study was to investigate whether and how target animacy (interaction with inanimate and animate targets) affected manual laterality in 12 zoo-housed Barbary macaques (Macaca sylvanus). We focused on both 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 and animate targets (during social and self-directed behaviors). and recorded social interactions to measure the rank. For inanimate targets, 14 15-minute sessions were done. For animate targets, 14 2-hour morning sessions and 14 90-minute afternoon sessions were done. 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. However, we found no group-level hand preference for actions directed toward inanimate or animate targets in general. However, we found a group-level right-hand bias for affiliative behaviors, whereas no effects of social rank on lateralization was present. Based on our results, both hemispheres seem to process emotions, influencing hand preference during social interactions, as suggested by the valence hypothesis. Despite the limitations due to the small sample size, these findings suggest that Barbary macaques show hand preference patterns similar to those found in great apes and other catarrhines in interactions with both inanimate and animate targets