Decoding the baboon's expressive capacity: facial microanatomy and the development of a PapioFACS

Facial mobility in primates reflects a dynamic interplay between anatomy and behavior, yet comparative data remain limited for many cercopithecines. Here, we present the first integrated anatomical and behavioral analysis of the facial mask in Papio hamadryas, combining gross dissection, histological analysis, and behavioral coding to develop a species-specific Facial Action Coding System (PapioFACS). Gross and histological analyses of five adult craniofacial specimens (10 hemifaces) identified 15 facial muscles, 3 extrinsic auricular muscles, and 4 intrinsic auricular muscles, and revealed a distinctive combination of anatomical features in this species. The auricular region was particularly well developed, whereas the perioral region showed a less differentiated organization: the nasolabial region consisted of a continuous maxillonasolabialis, the labial commissure lacked a discrete modiolus, and the platysma showed specialized integration with the cheek pouch. Histological analysis further demonstrated a dense, fibrotic superficial musculoaponeurotic system that differs markedly from the human condition. Guided by these anatomical findings, PapioFACS identified 20 Action Units, 12 Action Descriptors, and 3 Ear Action Descriptors, with final inter-coder agreement exceeding 90%. Together, these findings provide a species-specific framework for linking facial anatomy and facial movement in P. hamadryas and broaden the comparative basis for studying primate facial communication.