WILLIAMS, S.H.; VINYARD, C.J.; WALL, C.E.; HYLANDER, W.L.; Duke University, Durham; Duke University, Durham; Duke University, Durham; Duke University, Durham: Masticatory strains in the mandibular corpus of selenodont artiodactyls

Although selenodont artiodactyls have been the subject of several theoretical studies of masticatory biomechanics, very little is known about in vivo loading regimes in the mandible of these animals during mastication. Here, we present strain data from the mandibular corpus of goats (Capra hircus) and alpacas (Lama pacos) to provide a basis for the interpretation of mandibular form in selenodont artiodactyls. Goats and alpacas are broadly similar among mammals in masticatory form yet differ in some aspects of mandibular morphology and jaw-muscle recruitment patterns. These differences suggest that mandibular loading patterns may differ between these species. Most notably, alpacas have a solidly ossified mandibular symphysis and a balancing-side deep masseter muscle that is recruited late relative to the other jaw muscles. Goats have an unfused, mobile symphysis and an early-acting balancing-side deep masseter muscle. In this study, rosette strain gages were attached to the mandible below the last premolar or first molar in both species to record both the magnitude and direction of principal strains during the mastication of hay. The strain data indicate that in both species the working-side corpus is twisted about its long axis and/or sheared perpendicular to its long axis during the power stroke. On the balancing-side, the corpus undergoes a combination of parasagittal and lateral transverse bending, although additional data will determine whether one pattern predominates in either species. Interestingly, in selenodont artiodactyls, there is no apparent link between symphyseal morphology and the magnitude of balancing-side to working-side bone strain as has been proposed for other mammals.