Loading mechanics in femora of tiger salamanders (\u3cem\u3eAmbystoma tigrinum\u3c/em\u3e) and tegu lizards (\u3cem\u3eTupinambis merianae\u3c/em\u3e): implications for the evolution of limb bone design

The limb bones of tetrapods exhibit a wide range of shapes and sizes. Because locomotion is one of the most frequent and demanding behaviors in which limbs are used, this diversity in limb structure and design is frequently attributed to variation in the mechanical loading patterns that bones experience during locomotion. Limb bones are usually able to withstand loads much higher than they would normally experience before they fail. This margin of protection is known as a \u27safety factor.\u27 High safety factors would provide limb bones substantial insurance against failure, but could also make limb bones more costly to grow, maintain, and transport. Research in this area has focused mainly on birds and mammals, animals that use upright limb posture; however, a limited number of studies on reptilian species, in which the limbs are held in a sprawling posture, have shown that their limb bone loading patterns differ substantially from those of birds and mammals. To clarify whether the bone loading patterns observed in non-avian reptiles are ancestral or derived conditions, bone loading data from an additional species would provide a critical perspective. Salamanders are an ideal outgroup (outside the amniote clade) from which such data can be obtained. Additionally, among reptiles, lizards are one of the most diverse groups and among the most (at least superficially) similar in body plan to salamanders. Sampling a lizard species from a different lineage than that previously examined could help to determine whether bone loading patterns are similar across the breadth of lizard taxa and distinct, as a whole, from those of birds and mammals. This study evaluates the loads on the limb bones of the tiger salamander (Ambystoma tigrinum) and the Argentine black and white tegu (Tupinambus merianae) during terrestrial locomotion using three-dimensional measurements of the ground reaction force (GRF) and hindlimb kinematics, anatomical measurements of the femur and hindlimb muscles, and in vivo measurements of bone strain (tegus only). Peak tensile bending stresses in the femur were generally below 15 MPa, which is fairly low compared to observations from other vertebrate lineages. Using mechanical property values collected from hardness tests, femoral safety factors were calculated to be greater than 12 for both taxa, much higher than those seen in birds or mammals (which range from 2 to 4). This was due mainly to lower levels of locomotor stresses rather than any difference in mechanical properties of the bone. Together with data from other amphibian and reptile lineages, these results suggest that low magnitude loading and high limb bone safety factors may have an ancient evolutionary history