21.4 Sunday, Jan. 4 14:15 Mixed chains of safety factors in the limb bones of salamanders: implications for differential limb function in the evolution of terrestrial locomotion KAWANO, S.M.*; BLOB, R.W.; NIMBioS; Clemson Univ. email@example.com
The capacity of vertebrate bones to resist loads has important implications for their functional capabilities. Bones can often resist loads that are higher than they normally experience through an extra load bearing capacity, called a safety factor (SF). Alexander’s “mixed-chain” hypothesis proposed that different SFs might be found among “links” within a biological system (“chain”) as an adaptation to help accommodate unpredictable loads or variation in the energetic costs of elements, or if SFs were generally high across body structures. Data on the presence of “mixed chains” in vertebrate limbs are sparse; however, understanding the generality of “mixed chains” of limb bone SFs could shed light on a long-standing question of how different functional roles of forelimbs and hind limbs could have contributed to the invasion of land. We compared the mechanical properties and locomotor loading of the humerus and femur of tiger salamanders (Ambystoma tigrinum) in relation to the “mixed-chain” hypothesis, in order to assess the conditions under which functional diversity in SFs might emerge. Although the forelimbs and hind limbs appear similar in A. tigrinum, bone stresses in the humerus were roughly half (and SFs twice as great as) those observed in the femur, in part due to differences in muscle arrangements between the limbs. Also, regional heterogeneity in bone mechanical properties contributed to larger hardness values in the dorsal and posterior regions of both bones. Such intraspecific variation between and within bones may relate to the different biomechanical functions of these locomotor modules, and could have facilitated the acquisition of novel locomotor capabilities during the evolutionary invasion of land by tetrapods.