Masticatory stress and the mechanics of “wishboning” in colobine jaws

摘要

Cercopithecoid monkeys experience rela-ntively high strains along the lingual aspect of the mandib-nular symphysis because of lateral transverse bending ofnthe mandibular corpora (‘‘wishboning’’) during mastica-ntion. Hylander (Am J Phys Anthropol 64 (1984) 1–46; AmnZool 25 (1985) 315–330) demonstrated that the distribu-ntion of strains arising from wishboning loads is compre-nhensible with reference to the mechanics of curved beams.nTheory of curved beams suggests that lingual tensilenstrains are some multiple of labial compressive strains,nyet limitations of experimental methods and uncertaintynin estimating parameters needed for theoretical calcula-ntions have confounded attempts to characterize the mag-nnitude of this disparity of normal strains. We evaluate thentheoretical disparity of normal strains in wishboning inncomparison to in vitro strains collected under controllednloads for a sample of mandibles representing two colobinenspecies (N 5 6). These data suggest that in colobine mon-nkeys, maximum normal lingual strains should be at leastntwice maximum labial strains. In addition, we reexaminenthe distribution of symphyseal stress under an assump-ntion of asymmetric bending, a general approach for calcu-nlation of stress appropriate for members that lack a planenof symmetry and are bent along an axis that is not coinci-ndent with the member’s principal axes. Under asymmetricnbending in colobine mandibles, the effect of symphysealninclination on lingual strain is mitigating at the superiorntransverse torus and exacerbating at the inferior trans-nverse torus. Relative compliance of colobine mandibularnbone further supports the hypothesis that the structuralnand material properties of the colobine mandibular sym-nphysis do not represent a morphological strategy for mini-nmizing masticatory strain