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

Area of Science:

• Primate anatomy
• Developmental biology
• Ethology

Background:

• Rhesus macaques (Macaca mulatta) are a key model for human social behavior evolution.
• They exhibit complex social hierarchies and facial displays.
• Ontogenetic changes in facial musculature and their link to behavior are poorly understood.

Purpose of the Study:

• To investigate developmental changes in gross facial musculature in rhesus macaques.
• To compare infant and adult facial muscle development.
• To explore potential relationships between muscle ontogeny and facial displays.

Main Methods:

• Dissection of facial masks from 13 infant/fetal rhesus macaques (Macaca mulatta).
• Comparison of infant musculature with adult rhesus macaque facial anatomy.
• Gross anatomical examination of extrinsic and intrinsic facial muscles.

Main Results:

• Infant rhesus macaques (Macaca mulatta) possess gracile, poorly developed extrinsic ear muscles and lower lip muscles.
• Muscles of the upper lip and nares are well-developed and robust in infants, similar to adults.
• The zygomaticus major muscle was absent in all fetal/infant samples examined.

Conclusions:

• Facial muscle development in rhesus macaques (Macaca mulatta) varies significantly with age.
• Underdeveloped muscles in infants may indicate adaptations for specific life stages or developmental processes.
• These findings contribute to understanding facial allometry and the ontogeny of primate facial communication.