Department of Biological Sciences, Youngstown State University, Youngstown, OH 44555, USA.
J Exp Biol. 2011 Aug 1;214(Pt 15):2631-40. doi: 10.1242/jeb.049544.
Terrestrial locomotion can impose substantial loads on vertebrate limbs. Previous studies have shown that limb bones from cursorial species of eutherian mammals experience high bending loads with minimal torsion, whereas the limb bones of non-avian reptiles (and amphibians) exhibit considerable torsion in addition to bending. It has been hypothesized that these differences in loading regime are related to the difference in limb posture between upright mammals and sprawling reptiles, and that the loading patterns observed in non-avian reptiles may be ancestral for tetrapod vertebrates. To evaluate whether non-cursorial mammals show loading patterns more similar to those of sprawling lineages, we measured in vivo strains in the femur during terrestrial locomotion of the Virginia opossum (Didelphis virginiana), a marsupial that uses more crouched limb posture than most mammals from which bone strains have been recorded, and which belongs to a clade phylogenetically between reptiles and the eutherian mammals studied previously. The presence of substantial torsion in the femur of opossums, similar to non-avian reptiles, would suggest that this loading regime likely reflects an ancestral condition for tetrapod limb bone design. Strain recordings indicate the presence of both bending and appreciable torsion (shear strain: 419.1 ± 212.8 με) in the opossum femur, with planar strain analyses showing neutral axis orientations that placed the lateral aspect of the femur in tension at the time of peak strains. Such mediolateral bending was unexpected for a mammal running with near-parasagittal limb kinematics. Shear strains were similar in magnitude to peak compressive axial strains, with opossum femora experiencing similar bending loads but higher levels of torsion compared with most previously studied mammals. Analyses of peak femoral strains led to estimated safety factor ranges of 5.1-7.2 in bending and 5.5-7.3 in torsion, somewhat higher than typical mammalian values for bending, but approaching typical reptilian values for shear. Loading patterns of opossum limb bones therefore appear intermediate in some respects between those of eutherian mammals and non-avian reptiles, providing further support for hypotheses that high torsion and elevated limb bone safety factors may represent persistent ancestral conditions in the evolution of tetrapod limb bone loading and design.
陆地运动可能会对脊椎动物的四肢造成巨大的负荷。先前的研究表明，来自真兽类哺乳动物的奔跑物种的肢骨承受着高弯曲负荷，而扭转最小，而非鸟类爬行动物（和两栖动物）的肢骨除了弯曲之外还会产生相当大的扭转。据推测，这种负荷模式的差异与直立哺乳动物和伸展爬行动物之间的肢体姿势差异有关，并且在非鸟类爬行动物中观察到的负荷模式可能是四足脊椎动物的祖先。为了评估非奔跑哺乳动物是否表现出更类似于伸展谱系的负荷模式，我们在弗吉尼亚负鼠（Didelphis virginiana）的陆地运动过程中测量了股骨的体内应变，弗吉尼亚负鼠的四肢姿势比大多数记录有骨骼应变的哺乳动物更为蜷缩，并且属于爬行动物和之前研究的真兽类哺乳动物之间的进化枝。负鼠股骨中存在大量扭转，类似于非鸟类爬行动物，这表明这种负荷模式可能反映了四足脊椎动物肢骨设计的祖先条件。应变记录表明，负鼠股骨中存在弯曲和可观的扭转（剪切应变：419.1 ± 212.8 με），平面应变分析显示中性轴方向在峰值应变时使股骨的外侧处于拉伸状态。对于一种以近矢状肢运动学奔跑的哺乳动物来说，这种侧向弯曲是出乎意料的。剪切应变与峰值压缩轴向应变的大小相似，与大多数先前研究的哺乳动物相比，负鼠股骨承受相似的弯曲负荷，但扭转水平更高。股骨峰值应变的分析导致弯曲的估计安全系数范围为 5.1-7.2，扭转的为 5.5-7.3，略高于典型哺乳动物的弯曲值，但接近典型爬行动物的剪切值。因此，负鼠四肢骨骼的负荷模式在某些方面介于真兽类哺乳动物和非鸟类爬行动物之间，为高扭转和升高的肢骨安全系数可能代表四足脊椎动物肢骨负荷和设计进化中的持续祖先条件的假说提供了进一步支持。
