Rowland Institute, Harvard University, 100 Edwin H. Land Boulevard, Cambridge, MA 02142, USA.
J Exp Biol. 2011 Sep 15;214(Pt 18):3013-20. doi: 10.1242/jeb.059345.
Geometric scaling predicts that stresses on limb bones and muscles should increase with body size. Mammals counter this size-related increase in stress partially through changes in bone geometry, but largely through changes in posture, with larger species having a more erect stance. However, the ability to counter size-related stresses in this fashion may be limited to those taxa that have a parasagittal gait (such as mammals), where legs are swung underneath the body. We examined locomotor kinematics for 11 species of varanid lizards (from 0.04 to 8 kg body mass) that have a sprawling gait, to determine how they moderate size-related stresses. Posture, as indicated by femur adduction and hip heights, did not change significantly with body size, beyond that expected from geometrical scaling. Instead, lizards mitigated size-related increases in stress by increasing duty factor and possibly reducing femur rotation. Incorporating these factors in biomechanical models predicted that both bending (∝M(0.016), where M is mass) and torsional (∝M(-0.049)) stresses should be nearly independent of body size over the size range examined. However, increasing duty factor and reducing femur rotation probably have deleterious effects on speed, and this difference in kinematics with size may explain why speed scales lower for sprawling lizards than for parasagittal mammals (∝M(0.17) and ∝M(0.24), respectively). Further, paralleling conclusions for the synapsid lineage, these findings suggest that evolution from sprawling to upright posture did not occur in archosaurs as a response to larger size; rather, these archosaurs likely became upright first and larger later.
几何比例预测肢体骨骼和肌肉的应力应随体型增大而增加。哺乳动物通过改变骨骼几何形状来部分抵消这种与体型相关的压力增加，但主要通过改变姿势来实现，体型较大的物种通常采取更直立的姿势。然而，以这种方式抵消与体型相关的压力的能力可能仅限于那些具有矢状步态（如哺乳动物）的类群，在这种步态中，腿部在身体下方摆动。我们研究了 11 种巨蜥（体重范围为 0.04 至 8 千克）的运动学，这些巨蜥具有匍匐步态，以确定它们如何调节与体型相关的压力。除了由几何比例预测的外，股骨内收和髋骨高度所表示的姿势并没有随着体型的显著变化而变化。相反，蜥蜴通过增加任务因子并可能减少股骨旋转来减轻与体型相关的压力增加。将这些因素纳入生物力学模型预测，弯曲（∝M(0.016)，其中 M 是质量）和扭转（∝M(-0.049)）应力都应该在研究的体型范围内几乎与体型无关。然而，增加任务因子和减少股骨旋转可能对速度产生有害影响，这种体型大小的运动学差异可能解释了为什么匍匐蜥蜴的速度比矢状哺乳动物的速度要低（分别为∝M(0.17)和∝M(0.24)）。此外，与合弓类动物的结论类似，这些发现表明，从匍匐姿势到直立姿势的进化并不是恐龙为了应对更大的体型而发生的；相反，这些恐龙可能首先变得直立，后来体型才变大。
