Bone histology reveals life history and regional remodeling in black bears

Black bears (Ursus americanus) undergo prolonged inactivity during hibernation without the cortical bone loss typical of other mammals, yet the response of axial bones to this process remains understudied. This study compares midshaft femoral and rib histology in four individuals—three wild and one captive—to assess skeletal responses to hibernation, growth, and stress. Femora exhibit a laminar-plexiform woven–parallel complex (WPC), with parallel-fibered annuli bounded by lines of arrested growth (LAGs). The captive bear displayed an outer circumferential layer (OCL) after the seventh LAG, indicating skeletal maturity. One wild female with mange showed thick annuli and extensive remodeling, suggesting chronic physiological stress. Despite likely not hibernating, the captive bear's femoral histology closely resembled that of wild hibernators, suggesting conserved growth dynamics. Relative Cortical Area was consistently higher in femora than ribs, reflecting greater structural demands. Femoral remodeling was localized around the linea aspera, while ribs were heavily remodeled with enlarged resorption cavities and expanded trabecular networks. These results are consistent with region-specific skeletal responses and possible calcium mobilization during hibernation. Overall, bears appear to employ a region-specific skeletal strategy, maintaining cortical integrity in weight-bearing limbs while remodeling axial elements for calcium mobilization. This dual pattern highlights key insights into mammalian bone plasticity under seasonal metabolic stress.