The roles of phylogeny, body size and substrate use in trabecular bone variation among Philippine 'earthworm mice' (Rodentia: Chrotomyini)

Trabecular bone is modelled throughout an animal's life in response to its mechanical environment, but like other skeletal anatomy, it is also subject to evolutionary influences. Yet the relative strengths of factors that affect trabecular bone architecture are little studied. We investigated these influences across the Philippine endemic murine rodent clade Chrotomyini. These mammals have robustly established phylogenetic relationships, exhibit a range of well-documented substrate-use types, and have a body size range spanning several hundred grammes, making them ideal for a tractable study of extrinsic and intrinsic influences on trabecular bone morphology. We found slight differences in vertebral trabecular bone among different substrate-use categories, with more divergent characteristics in more ecologically specialized taxa. This suggests that the mechanical environment must be relatively extreme to affect trabecular bone morphology in small mammals. We also recovered allometric patterns that imply that selective pressures on bone may differ between small and large mammals. Finally, we found high intrataxonomic variation in trabecular bone morphology, but it is not clearly related to any variable we measured, and may represent a normal degree of variation in these animals rather than a functional trait. Future studies should address how this plasticity affects biomechanical properties and performance of the skeleton.