Abstract
Mandible morphology has yet to yield definitive information on primate diet, probably because of poor understanding of mandibular loading and strain regimes, and overreliance on simple beam models of mandibular mechanics. We used a finite element model of a macaque mandible to test hypotheses about mandibular loading and strain regimes and relate variation in muscle activity during chewing on different foods to variation in strain regimes. The balancing-side corpus is loaded primarily by sagittal shear forces and sagittal bending moments. On the working side, sagittal bending moments, anteroposterior twisting moments, and lateral transverse bending moments all reach similar maxima below the bite point; sagittal shear is the dominant loading regime behind the bite point; and the corpus is twisted such that the mandibular base is inverted. In the symphyseal region, the predominant loading regimes are lateral transverse bending and negative twisting about a mediolateral axis. Compared with grape and dried fruit chewing, nut chewing is associated with larger sagittal and transverse bending moments acting on balancing- and working-side mandibles, larger sagittal shear on the working side, and larger twisting moments about vertical and transverse axes in the symphyseal region. Nut chewing is also associated with higher minimum principal strain magnitudes in the balancing-side posterior ramus; higher sagittal shear strain magnitudes in the working-side buccal alveolar process and the balancing-side oblique line, recessus mandibulae, and endocondylar ridge; and higher transverse shear strains in the symphyseal region, the balancing-side medial prominence, and the balancing-side endocondylar ridge. The largest food-related differences in maximum principal and transverse shear strain magnitudes are in the transverse tori and in the balancing-side medial prominence, extramolar sulcus, oblique line, and endocondylar ridge. Food effects on the strain regime are most salient in areas not traditionally investigated, suggesting that studies seeking dietary effects on mandible morphology might be looking in the wrong places.
Highlights
Mandible morphology has yet to yield definitive information on primate diet, probably because of poor understanding of mandibular loading and strain regimes, and overreliance on simple beam models of mandibular mechanics
Hypotheses about loading and deformation regimes underlie decisions about measurements made by functional morphologists attempting to link mandibular morphology to feeding behaviour and diet
We described the strain regimes in the corpus, ramus and symphysis associated with these loading and deformation regimes
Summary
Mandible morphology has yet to yield definitive information on primate diet, probably because of poor understanding of mandibular loading and strain regimes, and overreliance on simple beam models of mandibular mechanics. Hylander in which hypotheses about mandibular stress, strain, and deformation regimes (sensu Ross et al, 2011) were derived from in vivo bone strain data from small areas of the corpus and symphysis (Hylander, 1977, 1979a, 1979b, 1979c, 1981, 1984, 1985, 1986, 1988; Hylander et al, 1987) In these studies, the in vivo data were explained and interpreted using simple beam models and assumptions about the relative magnitudes and orientations of the forces acting on the mandible during feeding. This intensive comparative morphometric research has yet to uncover strong relationships between mandible form, feeding behavior and diet in living primates (Daegling, 2007b; Daegling et al, 2016; Daegling and Grine, 2006; Hylander, 1988; McGraw and Daegling, 2012, 2020; Ravosa et al, 2016; Ross and Iriarte-Diaz, 2019b; Taylor, 2002, 2006a)
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