Cracking the case: Differential adaptations to hard biting dominate cranial shape in rat-kangaroos (Potoroidae: Bettongia) with divergent diets.

Functional adaptation in the mammalian jaw is often best predicted by the hardest bites an animal makes. Therefore, even when closely related species have otherwise divergent diets, a shared biomechanically challenging resource should be reflected in similar adaptations to jaw biomechanics. We assessed this in two species of rat-kangaroos, whose otherwise divergent diets include the extremely tough-shelled seeds of Santalum spp. (sandalwood and/or quandong). We used geometric morphometrics to analyse cranial shape of 161 bettongs (Bettongia spp.), including all four extant species. We identified adaptations to higher bite forces in both species that crack open Santalum seeds. However, B. lesueur had shorter facial proportions, indicating higher mechanical advantage, while B. penicillata had a premolar morphology that likely focussed bites to a specific, reinforced position on the jaw. This represents an example of many-to-one mapping at the genus level. We also found differences in a subsample of captive northern bettongs compared to wild conspecifics, suggesting some role of phenotypic plasticity in shaping adult skulls. The large olfactory tracts of B. penicillata, that support search for underground fungi, might have constrained its cranium to retain longer proportions. Fungal abundance could potentially be an important consideration in identifying translocation sites for this species.