Department of Biological Sciences, Clemson University, Clemson, SC 29634, USA.
J Exp Biol. 2011 Aug 1;214(Pt 15):2616-30. doi: 10.1242/jeb.048801.
Skeletal elements are usually able to withstand several times their usual load before they yield, and this ratio is known as the bone's safety factor. Limited studies on amphibians and non-avian reptiles have shown that they have much higher limb bone safety factors than birds and mammals. It has been hypothesized that this difference is related to the difference in posture between upright birds and mammals and sprawling ectotherms; however, limb bone loading data from a wider range of sprawling species are needed in order to determine whether the higher safety factors seen in amphibians and non-avian reptiles are ancestral or derived conditions. Tegus (family Teiidae) are an ideal lineage with which to expand sampling of limb bone loading mechanics for sprawling taxa, particularly for lizards, because they are from a different clade than previously sampled iguanas and exhibit different foraging and locomotor habits (actively foraging carnivore versus burst-activity herbivore). We evaluated the mechanics of locomotor loading for the femur of the Argentine black and white tegu (Tupinambus merianae) using three-dimensional measurements of the ground reaction force and hindlimb kinematics, in vivo bone strains and femoral mechanical properties. Peak bending stresses experienced by the femur were low (tensile: 10.4 ± 1.1 MPa; compressive: -17.4 ± 0.9 MPa) and comparable to those in other reptiles, with moderate shear stresses and strains also present. Analyses of peak femoral stresses and strains led to estimated safety factor ranges of 8.8-18.6 in bending and 7.8-17.5 in torsion, both substantially higher than typical for birds and mammals but similar to other sprawling tetrapods. These results broaden the range of reptilian and amphibian taxa in which high femoral safety factors have been evaluated and further indicate a trend for the independent evolution of lower limb bone safety factors in endothermic taxa.
骨骼元素通常能够承受其通常负荷的数倍而不会屈服，这个比值被称为骨骼的安全系数。对两栖动物和非鸟兽脚类恐龙的有限研究表明，它们的肢骨安全系数比鸟类和哺乳动物高得多。有人假设这种差异与直立鸟类和哺乳动物与伸展的变温动物之间的姿势差异有关；然而，需要更广泛的伸展物种的肢骨负荷数据，以确定在两栖动物和非鸟兽脚类恐龙中看到的更高安全系数是祖先条件还是衍生条件。巨蜥（Teiidae 科）是一个理想的进化枝，可用于扩大对伸展分类群的肢骨负荷力学的采样，特别是对于蜥蜴，因为它们与以前采样的鬣蜥不同，并且表现出不同的觅食和运动习惯（主动觅食的肉食动物与爆发性食草动物）。我们使用地面反作用力和后肢运动学的三维测量、体内骨应变和股骨机械特性，评估了阿根廷黑白巨蜥（Tupinambus merianae）股骨的运动加载力学。股骨经历的峰值弯曲应力较低（拉伸：10.4 ± 1.1 MPa；压缩：-17.4 ± 0.9 MPa），与其他爬行动物相似，也存在中等的剪切应力和应变。对股骨峰值应力和应变的分析导致弯曲时的估计安全系数范围为 8.8-18.6，扭转时为 7.8-17.5，均远高于鸟类和哺乳动物的典型值，但与其他伸展四足动物相似。这些结果扩大了评估股骨高安全系数的爬行动物和两栖动物的范围，并进一步表明了温血动物下肢骨安全系数独立进化的趋势。
