Functional Morphology of Mimetic Musculature in Primates: How Social Variables and Body Size Stack up to Phylogeny.

Mammalian skeletal muscle is influenced by the functional demands placed upon it. Functional morphology of facial expression musculature, or mimetic musculature, is largely unknown. Recently, primate mimetic musculature has been shown to respond to demands associated with social factors. Body size has also been demonstrated to affect many aspects of primate functional morphology and evolutionary morphology. The present study was designed to further examine the role of social variables and body size in influencing the morphology of primate mimetic musculature using a broad phylogenetic range of primates, primates with varying body sizes, and those that exploit differing time of day activity cycles and social group sizes. Gross data on mimetic musculature morphology were gathered from tarsiers (Tarsius bancanus), slender lorises (Loris tardigradus), ring-tailed lemurs (Lemur catta), crowned lemurs (Eulemur coronatus), black lemurs (E. macaco), owl monkeys (Aotus trivirgatus), and howler monkeys (Alouatta caraya) and compared to previous results from chimpanzees (Pan troglodytes), gibbons and siamangs (hylobatids), rhesus macaques (Macaca mulatta), Sulawesi macaques (M. nigra), common marmosets (Callithrix jacchus), and greater bushbabies (Otolemur spp.). Mimetic muscle presence/absence was observed and recorded. Results revealed that phylogenetic position determines the overall mimetic muscle groundplan, with anthropoids having a high number of muscles in the superciliary and midface regions, strepsirrhines having a high number of muscles in the external ear region, and tarsiers displaying an intermediate condition. Within these broad taxonomic categories body size had an effect on mimetic musculature, while time of day activity and social group size had smaller effects. Anat Rec, 301:202-215, 2018. © 2018 Wiley Periodicals, Inc.