VINYARD, Christopher J; WALL, Christine E*; WILLIAMS , Susan H; JOHNSON , Kirk R; HYLANDER, William L; NEOUCOM; Duke University; Ohio University; Duke University; Duke University: Are jaw-muscle activity patterns correlated with masticatory apparatus morphology among primate species?
A long-standing hypothesis in studies of primate mastication argues that morphological changes in the masticatory apparatus have evolved within a conserved behavioral framework. Thus, jaw-muscle activity patterns are thought to vary little across primate species with one pattern purportedly describing strepsirrhines and a second characterizing anthropoids. Until now, we have lacked sufficient in vivo data on jaw-muscle activity patterns during mastication to adequately test this hypothesis. We compared the recruitment and firing patterns of the superficial and deep masseter as well as the anterior and posterior temporalis muscles to measures of jaw and tooth form in Otolemur sp., Lemur catta, Propithecus verreauxi, Callithrix jacchus, Aotus trivirgatus, Macaca sp., Papio anubis and Tupaia belangeri. The above hypothesis predicts little association between jaw-muscle activity and morphology. Morphological estimates of relative symphyseal strength (Beecher, 1974) are significantly inversely correlated with the average working-to balancing-side (W/B) ratio for these jaw muscles (rs=-0.93) and the deep masseter W/B ratio (rs=-0.97) across these species. Thus, primate species with relatively greater symphyseal strength estimates tend to increase the relative recruitment of their balancing-side muscles during mastication. Additionally, the timing of peak firing for the balancing-side posterior temporalis is correlated with the ratio of cristid obliqua length (as an estimate of molar shearing) and M2 crushing area suggesting that specific jaw-muscle firing patterns are linked to dental form and function. Primate jaw-muscle activity patterns appear to be an integrated and evolving component of the masticatory apparatus. Supported by NSF.
