First evidence of population-level oro-facial asymmetries during the production of distress calls by macaque (Macaca mulatta) and baboon (Papio anubis) infants.

Infant distress calls are vocal communicative signals present in most animals. In nonhuman primates, they correspond to critical vocalizations for caregiving and contribute to the socio-emotional development of the individual. To our knowledge, no systematic study on the development of oro-facial hemispheric specialization in nonhuman primates infants is available. Thus, we proposed to assess to what extend emotional behaviors underlying distress calls in macaques and in baboons younger than 1 year of age may express lateralization. For the first time, a population-level cerebral lateralization was found for screaming and cooing calls in macaques and for the moaning call in baboons. However, differences in patterns of lateralization were found between the two vocalizations produced by macaques (for cooing, the left-side of the mouth opened widest than the right one and for screaming, a preference toward the right side of the mouth was noticed) as well as a sex effect for cooing. Our findings are discussed within the comparative literature in order to comprehend the ontogenetic and phylogenetic changes of hemispheric specialization for emotions in the primate order.