Testing hypotheses of skull function with comparative finite element analysis: three methods reveal contrasting results

Comparative finite element analysis often involves standardising aspects of models to test equivalent loading scenarios across species. However, regarding feeding biomechanics of the vertebrate skull, what is considered 'equivalent' can depend on the hypothesis. Using 13 diversely shaped skulls of marsupial bettongs and potoroos (Potoroidae), we demonstrate that scaling muscle forces to standardise specific aspects of biting mechanics can produce clearly opposing comparisons of stress or strain that are differentially suited to address specific kinds of hypotheses. We therefore propose three categories of hypotheses for skull biting mechanics, each involving a unique method of muscle scaling to produce meaningful results: those comparing (1) the skull's efficiency in distributing muscle forces to the biting teeth, via standardising input muscle force to skull size, (2) structural biting adaptation through standardising mechanical advantage to simulate size-adjusted, equivalent bites and (3) feeding ecology affected by size, such as niche partitioning, via standardising bite reaction force.