S5.1-1 Sunday, Jan. 5 08:00 Diversity of Limb Bone Safety Factors for Locomotion in Terrestrial Vertebrates: Evolution and Mixed Chains BLOB, R.W.*; ESPINOZA, N.R.; BUTCHER, M.T.; LEE, A.H.; D\'AMICO, A.R.; BAIG, F.M.; SHEFFIELD, K.M.; Clemson Univ.; Clemson Univ.; Youngstown St. Univ.; Midwestern Univ.; Clemson Univ.; Clemson Univ. firstname.lastname@example.org
During locomotion over land, vertebrate limb bones are exposed to loads. Like most biological structures, limb bones have a capacity to withstand greater loads than they usually experience, termed a safety factor (SF). How diverse are limb bone SFs, and what factors correlate with such variation? We have examined these questions from two perspectives. First, we used bone strain and force platform recordings to evaluate locomotor SF for the femur of diverse lineages, including salamanders, frogs, turtles, lizards, crocodilians, and marsupials (opossums). Comparisons with values for hind limb elements in running birds and eutherian mammals indicate phylogenetic diversity in limb bone SF. A high SF (>5) is primitive for tetrapods, but low load magnitudes and elevated bone strength contribute to different degrees across lineages. Birds and eutherians appear to have evolved lower SFs independently, a conclusion strengthened by finding SFs in opossums intermediate between those of eutherians and non-avian reptiles. Second, we tested the hypothesis that SFs would be similar across limb bones within a taxon by comparing the femur and humerus of alligators. In both bending and torsion, we found a higher SF for the humerus than the femur. Such a “mixed chain” of different SFs across elements has been predicted if bones have differing load variabilities, different costs to maintain, or high SF values. Though load variability is similar for the humerus and femur, a high SF may be less costly for the humerus because it is smaller than the femur. Moreover, the generally high SFs of alligators might promote SF differences among their limb bones.