Abstract
The mammalian jaw is often modelled as a third-class lever for the purposes of biomechanical analyses, owing to the position of the resultant muscle force between the jaw joint and the teeth. However, it has been proposed that in some rodents the jaws operate as a second-class lever during distal molar bites, owing to the rostral position of the masticatory musculature. In particular, the infraorbital portion of the zygomatico-mandibularis (IOZM) has been suggested to be of major importance in converting the masticatory system from a third-class to a second-class lever. The presence of the IOZM is diagnostic of the hystricomorph rodents, and is particularly well-developed in Pedetes capensis, the South African springhare. In this study, finite element analysis (FEA) was used to assess the lever mechanics of the springhare masticatory system, and to determine the function of the IOZM. An FE model of the skull of P. capensis was constructed and loaded with all masticatory muscles, and then solved for biting at each tooth in turn. Further load cases were created in which each masticatory muscle was removed in turn. The analyses showed that the mechanical advantage of the springhare jaws was above one at all molar bites and very close to one during the premolar bite. Removing the IOZM or masseter caused a drop in mechanical advantage at all bites, but affected strain patterns and cranial deformation very little. Removing the ZM had only a small effect on mechanical advantage, but produced a substantial reduction in strain and deformation across the skull. It was concluded that the masticatory system of P. capensis acts as a second class lever during bites along almost the entire cheek tooth row. The IOZM is clearly a major contributor to this effect, but the masseter also has a part to play. The benefit of the IOZM is that it adds force without substantially contributing to strain or deformation of the skull. This may help explain why the hystricomorphous morphology has evolved multiple times independently within Rodentia.
Highlights
Removal of either the infraorbital portion of the zygomaticomandibularis (IOZM) or the masseter causes a decrease in mechanical advantage during both incision and mastication, with removal of the IOZM leading to the greatest decrease
Removal of the medial pterygoid muscle leads to an increase in mechanical advantage across all cheek teeth, but little effect is seen during incisor biting
The skull of Pedetes capensis operates as a second-class lever during biting along almost all of the cheek teeth, and this effect can be largely ascribed to the presence of the IOZM muscle, the masseter is important in this regard as well
Summary
The mammalian jaw is frequently treated as a lever for the purposes of biomechanical analysis (e.g., Crompton, 1963; Bramble, 1978; Greaves, 1978; Greaves, 1982; Greaves, 2000; Gingerich, 1979; Thomason, 1991; Satoh, 1998; Satoh, 1999; Spencer, 1998; Spencer, 1999; Satoh & Iwaku, 2006; Satoh & Iwaku, 2009; Davis et al, 2010; Druzinsky, 2010; Cornette et al, 2012; Becerra, Casinos & Vassallo, 2013; Santana, 2015). The trade-off is that the mechanical advantage of a third-class lever is always less than one—that is, the output bite force will always be less than the effective muscle force
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