Interspecific scaling of the hindlimb skeleton in lizards, crocodilians, felids and canids: does limb bone shape correlate with limb posture?

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 predomi nantly in bending, but the limb bones of lizards and crocodilians are loade d primarily in torsion. As body size increases, torsional shear stress in l imb 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 c ompared with patterns in felid and canid mammals, using both non-phylogenet ic statistical methods and phylogenetically independent contrasts. Comparis ons with theoretical models indicate that size-related changes in limb bone geometry do not completely compensate for size-related increases in limb b one stress in the lizard or crocodilian lineages examined. Unless lizards a nd crocodilians compensate for size-related increases in limb bone stress t hrough other mechanisms (e.g. changes in limb kinematics or the mechanical properties of limb bones), limb bone stresses are predicted to be relativel y greater among larger species of these lineages. However, limb bone diamet ers appear to scale with greater positive allometry (relative to body mass) in varanids than in iguanians, suggesting that larger lizard lineages migh t compensate for increased stress through changes in bone geometry to a gre ater degree than smaller lineages. Allometric scaling patterns for many lim b bone diameters among iguanians are more similar to those of felids and ca nids than to those of varanids; thus, sprawling locomotor habits do not cor relate clearly with a particular pattern of limb bone scaling. This suggest s that similarity of interspecific scaling patterns of limb bone lengths an d diameters is not sufficient to justify inferences of similar locomotor fu nction.