Ontogeny of Rhesus Macaque Facial Musculature from the Fetal Period to Infancy: The Intersection of Anatomy and Social Behavior

Abstract Body : Rhesus macaques (Macaca mulatta) are a common animal model for the evolution of some aspects of human social behavior. Rhesus macaques have a rigid, despotic dominance hierarchy that pairs with a well-understood facial display repertoire. However, we know little about the ontogenetic changes in gross facial muscles and the development of facial display behavior. Because rhesus macaques are despotic in their social behavior, meaning that offspring of high ranking females have preferential access to resources, and that they have a well-established facial display repertoire, we hypothesized that rhesus macaque facial musculature form is established at least by birth. We dissected face masks from 12 M. mulatta cadavers that died naturally, were spontaneously aborted, or were stillbirths, to test this hypothesis. We made a midline incision using a scalpel beginning at the dorsal cervical region and passing through the occipital, frontal, nasal, and upper/lower lip regions. The left side of the face was removed to reveal facial muscles. These masks were set aside to air dry for increased color contrast between musculature and connective tissue prior to further analysis. Connective tissue was then removed in sequential stages from the masks using micro-dissection tools and lighted, surgical loupes. Musculature present was noted for each specimen as well as the attachments of the muscles. Results showed that, unlike adult M. mulatta, infant and fetal rhesus macaques have exceptionally gracile and poorly developed muscles of the extrinsic external ear and lower lip. Musculature associated with the upper lip and nares, including the depressor septi muscle, the orbicularis oris muscle, and the leavtor labii superioris muscle, were well developed, individually distinct, and robust. In addition, several muscles associated with the medial aspect of the superciliary region were well developed, including the procerus and corrugator supercilii muscles. While these results are preliminary, they are intriguing in light of the fact that neonatal rhesus macaques imitate adult facial displays as early as 10 days of postnatal age. Future work focusing on microanatomy of the fetal and infant M. mulatta facial muscles will no doubt illuminate further functional and ontogenetic aspects of rhesus macaque social behavior.