In femora from rats or mice, the area and moment of inertia (but not the wall-lumen ratio) of a diaphyseal section correlated with biomass and were determinants of the strength and stiffness of the integrated bone. In otter metacarpals, however, the geometric variable typically associated with body weight and mechanical ability of the integrated bone was the wall-lumen ratio (not the sectional moment of inertia). These differences may be associated with the meaningfulness of body density for natural selection in diving species like otters. A negative relationship between wall-lumen ratio and material modulus of elasticity in bones from the three species pointed out the impossibility of increasing diaphyseal thickness and stiffness at the same time. The results are compatible with the hypothesis that ecologic habits are selectively more important than phylogenetic relationships between species for the determination of bone mechanical properties in the upper vertebrates.