During locomotion, lizards and crocodilians generally use a more sprawling limb posture than most mammals and experience substantial axial rotation of the femur. Consequently, the limb bones of most mammals are loaded predominantly in bending, but the limb bones of lizards and crocodilians are loaded primarily in torsion. As body size increases, torsional shear stress in limb bones is expected to increase more than bending stress; therefore, limb bone diameters of lizards and crocodilians might be expected to scale with relatively greater positive allometry than limb bone diameters of mammals that use upright posture. To test this hypothesis, scaling patterns of the femur and tibia in lizards (iguanians and varanids) and crocodilians were compared with patterns in felid and canid mammals, using both non-phylogenetic statistical methods and phylogenetically independent contrasts. Comparisons with theoretical models indicate that size-related changes in limb bone geometry do not completely compensate for size-related increases in limb bone stress in the lizard or crocodilian lineages examined. Unless lizards and crocodilians compensate for size-related increases in limb bone stress through other mechanisms (e.g. changes in limb kinematics or the mechanical properties of limb bones), limb bone stresses are predicted to be relatively greater among larger species of these lineages. However, limb bone diameters appear to scale with greater positive allometry (relative to body mass) in varanids than in iguanians, suggesting that larger lizard lineages might compensate for increased stress through changes in bone geometry to a greater degree than smaller lineages. Allometric scaling patterns for many limb bone diameters among iguanians are more similar to those of felids and canids than to those of varanids; thus, sprawling locomotor habits do not correlate clearly with a particular pattern of limb bone scaling. This suggests that similarity of interspecific scaling patterns of limb bone lengths and diameters is not sufficient to justify inferences of similar locomotor function.
