Development of Rhesus Macaque (<i>Macaca mulatta</i>) Facial Musculature: Implications for Macaque Social Behavior Ontogeny

Rhesus macaques (Macaca mulatta) have long been used as a model for the evolution of some aspects of human social behavior and they are among the most completely understood species of the macaque genus. Rhesus macaques have a despotic dominance hierarchy with a well-documented facial display repertoire. However, we know little about the ontogenetic changes in gross facial musculature and how this might relate to corresponding facial display behavior. In an effort to illuminate our understanding, we dissected facial masks from 13 M. mulatta cadavers that died naturally, shortly after birth or were stillborn, and we compared these results to those from adult rhesus macaques. Results reveal that, unlike the adults in the present study, infant rhesus macaques have extrinsic external ear muscles and muscles associated with the lower lip that are very gracile and poorly developed. Musculature associated with the upper lip and nares are well developed, individually distinct, and robust, like those of the adults in our sample and adult M. mulatta. However, we were unable to locate the zygomaticus major muscle in any fetal/infant sample. These results may reflect simple variation in mimetic muscle ontogeny, differences related to facial allometry, or they could reflect a behaviorally meaningful adaptation to different life stages of macaque ontogeny.