Comparative finite element analysis of the cranial performance of four herbivorous marsupials

Abstract

Marsupial herbivores exhibit a wide variety of skull shapes and sizes to exploit different ecological niches. Several studies on teeth, dentaries, and jaw adductor muscles indicate that marsupial herbivores exhibit different specializations for grazing and browsing. No studies, however, have examined the skulls of marsupial herbivores to determine the relationship between stress and strain, and the evolution of skull shape. The relationship between skull morphology, biomechanical performance, and diet was tested by applying the finite element method to the skulls of four marsupial herbivores: the common wombat (Vombatus ursinus), koala (Phascolarctos cinereus), swamp wallaby (Wallabia bicolor), and red kangaroo (Macropus rufus). It was hypothesized that grazers, requiring stronger skulls to process tougher food, would have higher biomechanical performance than browsers. This was true when comparing the koala and wallaby (browsers) to the wombat (a grazer). The cranial model of the wombat resulted in low stress and high mechanical efficiency in relation to a robust skull capable of generating high bite forces. However, the kangaroo, also a grazer, has evolved a very different strategy to process tough food. The cranium is much more gracile and has higher stress and lower mechanical efficiency, but they adopt a different method of processing food by having a curved tooth row to concentrate force in a smaller area and molar progression to remove worn teeth from the tooth row. Therefore, the position of the bite is crucial for the structural performance of the kangaroo skull, while it is not for the wombat which process food along the entire tooth row. In accordance with previous studies, the results from this study show the mammalian skull is optimized to resist forces generated during feeding. However, other factors, including the lifestyle of the animal and its environment, also affect selection for skull morphology to meet multiple functional demands.

Individual variation of the masticatory system dominates 3D skull shape in the herbivory-adapted marsupial wombats

“…Our results support our "biomechanics hypothesis" of a non-allometric, mastication-related driver of cranio-mandibular shape variation within wombat species. As expected for a skull shaped by masticatory biomechanics [29,31,32,47], the heat plots of hypothetical PC1 shape extremes within species reveal strikingly uniform patterns of high landmark displacements in the zygomatic arches and rostra, also consistent with our hypothesis that these areas should vary most within species (in the mandible, displacement directions are not as uniform but still all occur in the muscle attachement sites). There is little indication that allometry plays any role in shaping within-species shape variation, with both overall shape and cranio-mandibular shape co-variation not related to centroid size (and shape co-variation instead strongly correlated with the main ordinated variation).…”

Variability, morphometrics, and co‐variation of the os lacrimale in Cervidae

“…In particular, in artiodactyls, the large lacrimal facial facet is stretched between the maxilla and the frontal bones and thus susceptible to mechanical loads transmitted from these bones. This seems to be supported by more recent finite‐element modeling studies that assessed stress‐transfer across the facial skeleton during feeding in suids and diprotodont marsupials (Bright, ; Sharp, ).…”

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

“…Muscle directions of pull (assuming a gape angle of 0°, i.e., teeth in occlusion) were assigned using landmarks recorded from the insertion areas on a reconstruction of the springhare mandible, created from the previously gathered microCT scans. Muscle forces were calculated by multiplying the physiological cross-sectional areas (PCSA) given in Offermans & De Vree (1993) by an intrinsic muscle stress value of 0.3 Nmm −2 (Van Spronsen et al, 1989; Sharp, 2015; Tseng & Flynn, 2015). These muscle forces were then modified based on the maximum percentage activations recorded by electromyography during incision and mastication of groundnuts (Offermans & De Vree, 1993).…”

“…To address the hypotheses and to study the function of the springhare skull during biting, finite element analysis (FEA) will be employed. FEA is an engineering technique for predicting stress, strain and deformation in an object during loading (Rayfield, 2007), and is now frequently applied to reconstructions of skulls and other skeletal elements in order to analyse vertebrate biomechanics (e.g., Richmond et al, 2005; Kupczik et al, 2007; Dumont et al, 2011; Ross et al, 2011; Cox et al, 2012; Cox, Kirkham & Herrel, 2013; O’Hare et al, 2013; Porro et al, 2013; Figueirido et al, 2014; Cuff, Bright & Rayfield, 2015; Sharp, 2015; McIntosh & Cox, 2016; McCabe et al, 2017; Tsouknidas et al, 2017). As well as simulating stress and strain distributions, FEA is also able to predict reaction forces, and so will be used here to estimate bite force, jaw joint reaction force and mechanical advantage.…”