ESPINOZA, N.R.**; CIRILO, S.R.; HILL, S.; BLOB, R.W.; Erskine College, Due West, SC; Erskine College, Due West, SC; Erskine College, Due West, SC; Clemson Univ., SC: Limb Bone Strain in Jumping Frogs
A primary requirement of limb bone design is the accommodation of loads experienced during locomotion. Patterns of limb bone loading have generally been viewed as highly conserved throughout tetrapod evolution, but most data have been derived from mammalian and avian species with a restricted range of locomotor styles. Limited data from reptilian lineages (crocodilians, lizards, turtles) have indicated patterns that differ from those of mammals and birds, with reptiles showing lower strain magnitudes but a greater prevalence of torsion. To further examine the diversity and evolution of limb bone loading patterns in tetrapods, and to test how distinctive modes of locomotion affect limb bone loads, we measured in vivo strains from multiple locations on the femur of jumping bullfrogs (Rana catesbeiana) using surgically implanted rosette and single element gauges. Because the jumps of ranid frogs are an explosive, discontinuous mode of locomotion, we predicted that strain magnitudes in jumping bullfrogs would be higher than those of previously tested reptiles and closer to levels found in mammals and birds. Instead, peak femoral strains in frogs were lower than those of mammals and birds and similar to those of reptiles, including slower moving turtles. Also similar to reptiles, torsional strains were substantial in the frog femur. Several mechanisms might contribute to the moderate strain magnitudes observed in frogs despite their dramatic jumps, including simultaneous activation of extensor muscles on opposite sides of the femur, or increased resistance to deformation through specialization of bone shape or material properties. Moreover, the similarity of loading magnitudes and regimes across amphibian and reptilian taxa suggests that the patterns seen in these lineages might be ancestral features of tetrapod limb bone design.
