The jaw is a second-class lever inPedetes capensis(Rodentia: Pedetidae)

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.

Masticatory musculature of the African mole-rats (Rodentia: Bathyergidae)

“…The loss of the rostral extension of the IOZM seems an unusual morphological change, given that this muscle is known to improve the efficiency of molar chewing in rodents (Cox et al, 2012;Cox, 2017). It is possible that it is an adaptation towards increased use of the incisors in digging, as has been suggested for H. glaber (Cox & Faulkes, 2014).…”

Functional morphology of the jaw adductor muscles in the Canidae

“…This muscle arrangement may also align with the more pronounced grinding function associated with the caudal teeth in these species. Possible future studies could consider running models with muscles sequentially removed, following previous studies (Cox, 2017;Cox, Kirkham, & Herrel, 2013) to establish how individual muscles contribute to bite force outputs.…”

Dental microwear texture analysis along reptile tooth rows: complex variation with non-dietary variables

“…the medial dorsal point on the articular ( figure 1 ). The mandible, plus a theoretical adductor muscle pulling the jaw closed, is viewed as a third-class lever, where the force exerted by muscles is located in between the fulcrum and the biting position [ 66 ]. Mechanical advantage was calculated using the equation where Force.Out denotes experienced bite force at the site of study in the mandible, Force.In denotes applied muscle force, in-lever is the horizontal distance from the jaw-joint to the mid-point of the adductor musculature and out-lever is the distance from the jaw-joint to the tooth of interest [ 49 , 50 ].…”