Overlooked role of bone quality in cranial adaptations to altered loading patterns

The effects of diet on craniomandibular form has been the focus of many functional, ecomorphological, and paleontological studies in mammals, with extensive work devoted to understanding the influence of adaptive plasticity. Prior research has established that dietary variability yields considerable morphological variation in bone quantity across cranial sites. However, the role of bone quality is less understood. Thus, there remains a significant gap in understanding how dietary properties affect nanoscale variation and, by extension, the hierarchical organization of bone in the developing skull and oral cavity, and whether bone quality (= tissue properties) covaries with bone quantity. Weanling white rabbits were divided equally between two dietary cohorts and raised until one-year old. Control rabbits were fed pellets. Overuse rabbits were given pellets and hay cubes for the duration, modeling a mechanically challenging diet that results in elevated cyclical loading. Nanoindentation was utilized to detail the stiffness and hardness of cortical bone post-mortem across masticatory and non-masticatory sites. Results indicate that diet-induced differences in loading affect nanoscale plasticity in masticatory elements without corresponding changes in the neurocranium, highlighting regional variation in responses to mechanical forces. The presence and magnitude of variation at the nanoscale vary according to masticatory site. This is critically important as it suggests that a functional signal may be site-specific, potentially posing an issue for accurate behavioral and biomechanical reconstructions. These findings highlight functional and developmental variation in determinants of cortical bone quality in the mammal skull and feeding complex, information of considerable utility for biomedical, ecomorphological, paleobiological, and evolutionary research.