Craniofacial Biomechanics of Primates

Duke University, Durham NC

Investigators

Abstract

Recently there has been renewed interest in determining the functional and adaptive significance of symphyseal fusion of the mandible in primates and other mammals. This interest is in part because symphyseal fusion is thought to play an important role in various adaptive scenarios of mammalian and especially primate evolution. This study tests the hypothesis that jaw muscle force recruitment and firing patterns vary between species of strepsirrhines, depending on the stiffness and strength of their symphyses. This hypothesis predicts that those strepsirrhine species such as the indriids, with their stiffer, stronger, and often partially-fused symphyses, have a jaw-muscle firing pattern that is similar to the derived condition found in living anthropoids, i.e., the "wishboning" pattern. Furthermore, this hypothesis also predicts that those strepsirrhines with a completely unfused and highly mobile symphysis, such as ring-tailed lemurs and thick-tailed galagos, do not have this pattern. Instead, they have the primitive mammalian motor pattern, which is also found in North American opossums and tree shrews. This hypothesis will be tested by analyzing masseter, medial pterygoid and temporalis muscle force recruitment and firing patterns using EMG techniques in (1) ring-tailed lemurs (Lemur catta), (2) gentle or bamboo lemurs (Hapalemur griseus), (3) sifakas (Propithecus verreauxi), and (4) aye-ayes (Daubentonia madagascariensis). It is predicted that EMG patterns in ring-tailed lemurs and aye-ayes will be more similar to patterns found in thick-tailed galagos, and that EMG patterns in bamboo lemurs and sifakas will be more similar to those patterns found in anthropoids. If the predictions for bamboo lemurs and sifakas are confirmed, this constitutes a refutation of the ontogenetic timing hypothesis regarding symphyseal fusion simply because strepsirrhines that exhibit late partial and/or complete fusion are hypothesized to not have the "wishboning" pattern of anthropoids. Furthermore, these data would also indicate that the "wishboning" pattern of anthropoids evolved more than once within primates, and if so, perhaps it evolved independently several times in various primate lineages. In summary, the main purpose of this project is to pursue these matters further by analyzing jaw-muscle force recruitment and firing patterns in various strepsirrhine species that have completely or partially unfused symphyses, but yet vary in the amount of symphyseal stiffness and strength