Of mice, monkeys, and men: Physiological and morphological evidence for evolutionary divergence of function in mimetic musculature

Facial expression is a universal means of visual communication in humans and many other primates. Humans have the most complex facial display repertoire among primates but gross morphological studies have not found greater complexity in human mimetic musculature. The present study examines microanatomical aspects of mimetic musculature in order to test hypotheses related to human mimetic musculature physiology, function, and evolutionary morphology. Samples from the orbicularis oris (OOM) and the zygomaticus major muscles (ZM) in laboratory mice (N=3), rhesus macaques (N=3) and humans (N=3) were collected. Fiber type proportions (slow-twitch and fast-twitch), fiber cross-sectional area, diameter, and length were calculated and means were statistically compared among groups. Results showed that macaques had the greatest percentage of fast fibers in both muscles (followed by humans) and humans had the greatest percentage of slow fibers in both muscles. Macaques and humans typically did not differ from one another in morphometrics except for fiber length where humans had longer fibers. While sample sizes are low, results from the present study may indicate that the rhesus macaque OOM and ZM are specialized primarily to assist with maintenance of the rigid dominance hierarchy via rapid facial displays of submission and aggression while human musculature may have evolved not only under pressure to work in facial expressions but also in development of speech.