Masticatory stress and the mechanics of “wishboning” in colobine jaws
Article first published online: 11 SEP 2008
Copyright © 2008 Wiley-Liss, Inc.
American Journal of Physical Anthropology
Volume 138, Issue 3, pages 306–317, March 2009
How to Cite
Daegling, D. J. and McGraw, W. S. (2009), Masticatory stress and the mechanics of “wishboning” in colobine jaws. Am. J. Phys. Anthropol., 138: 306–317. doi: 10.1002/ajpa.20929
- Issue published online: 29 JAN 2009
- Article first published online: 11 SEP 2008
- Manuscript Accepted: 5 AUG 2008
- Manuscript Received: 20 MAY 2008
- mandibular symphysis;
- asymmetric bending;
- curved beam
Cercopithecoid monkeys experience relatively high strains along the lingual aspect of the mandibular symphysis because of lateral transverse bending of the mandibular corpora (“wishboning”) during mastication. Hylander (Am J Phys Anthropol 64 (1984) 1–46; Am Zool 25 (1985) 315–330) demonstrated that the distribution of strains arising from wishboning loads is comprehensible with reference to the mechanics of curved beams. Theory of curved beams suggests that lingual tensile strains are some multiple of labial compressive strains, yet limitations of experimental methods and uncertainty in estimating parameters needed for theoretical calculations have confounded attempts to characterize the magnitude of this disparity of normal strains. We evaluate the theoretical disparity of normal strains in wishboning in comparison to in vitro strains collected under controlled loads for a sample of mandibles representing two colobine species (N = 6). These data suggest that in colobine monkeys, maximum normal lingual strains should be at least twice maximum labial strains. In addition, we reexamine the distribution of symphyseal stress under an assumption of asymmetric bending, a general approach for calculation of stress appropriate for members that lack a plane of symmetry and are bent along an axis that is not coincident with the member's principal axes. Under asymmetric bending in colobine mandibles, the effect of symphyseal inclination on lingual strain is mitigating at the superior transverse torus and exacerbating at the inferior transverse torus. Relative compliance of colobine mandibular bone further supports the hypothesis that the structural and material properties of the colobine mandibular symphysis do not represent a morphological strategy for minimizing masticatory strain. Am J Phys Anthropol, 2009. © 2008 Wiley-Liss, Inc.