The mechanical properties of the bone material of femora of five wild polar bears, ranging in age from three months to seven and three-quarter years, were compared with those of humans and axis deer. There are changes in mechanical properties in all three species with age, but their time course and extent vary greatly between species. The age-related changes in mechanical properties are attributable mainly to changes in mineralization. At any age, the properties of the polar bear's bones lie between those of humans and the deer. The architecture of the bear's bones, and the mechanical properties of their bone material, were related to the weight of the bears. The calculated stresses in the bones, and their deflection under load, were relatively constant compared with the very large differences in the weights of the bears and the sizes of their bones.