Human faces are slower than chimpanzee faces

• Type of Material: Article
• Authors: Anne M. Burrows, Duquesne University; Lisa Parr, Emory University; Emily L. Durham, Duquesne University; Lea C. Matthews, Duquesne University; Timothy D. Smith, University of Pittsburgh
• Language: English
• Date: 2014-10-22
• Publisher: Public Library of Science
• Publication Version: Final Published Version
• Copyright Statement: © 2014 Burrows et al.
• License: Creative Commons Attribution 4.0 International
• Final Published Version (URL): http://dx.doi.org/10.1371/journal.pone.0110523
• Title of Journal or Parent Work: PLoS ONE
• ISSN: 1932-6203
• Volume: 9
• Issue: 10
• Start Page: e110523
• End Page: e110523
• Grant/Funding Information: This study was supported by contract grant number MH082282 to lap (www.nih.gov); funding from the Samuel & Emma Winters Foundation to AMB (no website).
• Keywords: Chimpanzees; Macaque; Muscle Fibers, Slow-Twitch; Nonverbal Communication; Rhesus monkeys; Primates; Animals; Face; Tongue; Animal Communication; Muscle Contraction; Myosins; Slow-twitch muscle fibers; Pan troglodytes; Fast-twitch muscle fibers; Facial Muscles; Species Specificity; Speech; Facial Expression; Muscle Fibers, Fast-Twitch; Humans
• Research Categories: Psychology, Behavioral; Biology, Neuroscience

Abstract

Background: While humans (like other primates) communicate with facial expressions, the evolution of speech added a new function to the facial muscles (facial expression muscles). The evolution of speech required the development of a coordinated action between visual (movement of the lips) and auditory signals in a rhythmic fashion to produce "visemes" (visual movements of the lips that correspond to specific sounds). Visemes depend upon facial muscles to regulate shape of the lips, which themselves act as speech articulators. This movement necessitates a more controlled, sustained muscle contraction than that produced during spontaneous facial expressions which occur rapidly and last only a short period of time. Recently, it was found that human tongue musculature contains a higher proportion of slow-twitch myosin fibers than in rhesus macaques, which is related to the slower, more controlled movements of the human tongue in the production of speech. Are there similar unique, evolutionary physiologic biases found in human facial musculature related to the evolution of speech? Methodology/Prinicipal Findings: Using myosin immunohistochemistry, we tested the hypothesis that human facial musculature has a higher percentage of slow-twitch myosin fibers relative to chimpanzees (Pan troglodytes) and rhesus macaques (Macaca mulatta). We sampled the orbicularis oris and zygomaticus major muscles from three cadavers of each species and compared proportions of fiber-types. Results confirmed our hypothesis: humans had the highest proportion of slow-twitch myosin fibers while chimpanzees had the highest proportion of fast-twitch fibers. Conclusions/significance: These findings demonstrate that the human face is slower than that of rhesus macaques and our closest living relative, the chimpanzee. They also support the assertion that human facial musculature and speech coevolved. Further, these results suggest a unique set of evolutionary selective pressures on human facial musculature to slow down while the function of this muscle group diverged from that of other primates.