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; however, gross morphological studies have not found greater complexity in human mimetic musculature. This study examines the microanatomical aspects of mimetic musculature to test the hypotheses related to human mimetic musculature physiology, function, and evolutionary morphology. Samples from the orbicularis oris muscle (OOM) and the zygomaticus major (ZM) muscle 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 that 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. Although sample sizes are low, results from this study may indicate that the rhesus macaque OOM and ZM muscle are specialized primarily to assist with maintenance of the rigid dominance hierarchy via rapid facial displays of submission and aggression, whereas human musculature may have evolved not only under pressure to work in facial expressions but also in development of speech.