Comparative and functional anatomy of masticatory muscles and bite force in opossums (Didelphimorphia, Didelphidae)

Opossums (Didelphidae) are American marsupials traditionally known for their generalized morphology and generalist feeding habits. They include a diversity of similar items in their diets, but the proportion of types of items consumed varies between taxa. Thus, feeding ecology shows varying degrees of omnivory or food preference that cannot be distinguished into strict dietary categories. With few exceptions, the anatomical and functional relationship between the masticatory muscles and variation in food resources used in opossums is unknown. Here we provide comparative descriptions of the jaw adductor muscle anatomy and architecture of nine Didelphidae genera. The muscles were dissected, weighed, and chemically digested for separation and measurement of the muscle fascicles. We estimated the potential physiological cross-sectional area (pPCSA) of the muscles and used 2D lever arm mechanics to calculate the potential bite force on the canine and first molar. We tested the allometric relationships of muscle variables and bite forces and the correlation of bite forces with diet and diet mechanical challenge (relative frequency of hard items). The adductor muscles were represented by the m. temporalis and m. masseter, with two layers (superficial and deep) each, and m. pterygoideus medialis across the sample. The m. zygomaticomandibularis was also identified in most genera, except for Didelphis and Lutreolina. Muscle anatomy is conserved but varies in the extent of the attachment areas, in part due to differences in skull morphology. The anatomical diversification and relationships between muscles corresponded to a generalized pattern in most genera, which proved to be efficient for adding different items to the diet. The mass, average fascicle length, and pPCSA of the adductor muscles scaled isometrically with size. Bite forces on the canine and first molar also scaled isometrically and were not correlated with diet or diet mechanical challenge. Therefore, the variation in quantitative myological data and bite force was consistent with size, and the increase in bite force supports dietary diversification associated with increased size in opossums.