Cyclical loading, daily feeding modality and the saturation response in the developing skull.

Diet-induced cyclical loading causes elevated bony responses in mammal jaws, an adaptive phenomenon likewise observed between increased locomotor stresses and limb growth. Mammal limbs exhibit reduced mechanosensitivity and osteogenesis after prolonged cyclical loading, with rest periods between loading bouts mitigating this “saturation response”. This phenomenon has never been investigated in the skull despite documented variation in feeding modalities among diverse mammals. We have demonstrated, with a long-term dietary experiment, that domestic white rabbits raised unimodally (unrestricted feeding), developed less cortical bone than rabbits raised bimodally (two 2.25-h feeding periods separated by five hours rest), indicating a saturation response. Trimodal rabbits (three 1.5-h feeding periods separated by 2.5 h of rest) were intermediate, highlighting potential interactions between feeding modality and cyclical loading. Elevating dietary stiffness/toughness, which necessitates increasing chew cycles, did not alter overall patterns. Saturation responses do occur in the skull related to variation in daily feeding modality, but adaptive responses vary across the skull. Thus, ecomorphological relationships are complex, with multifactorial effects resident at disparate temporal and structural scales. The role of behavior-induced feeding plasticity on cranial development poses a challenge for paleobiological and computer models of skeletal function and provides a novel understanding of connective-tissue mechanobiology in vertebrates.