The crouching of the shrew: mechanical consequences of limb posture in small mammals

An important trend in the early evolution of mammals was the shift from a sprawling stance, whereby the legs are held in a more abducted position, to a parasagittal one, in which the legs extend more downward. After that transition, many mammals shifted from a crouching stance to a more upright one. It is hypothesized that one consequence of these transitions was a decrease in the total mechanical power required for locomotion, because side-to-side accelerations of the body have become smaller, and thus less costly with changes in limb orientation. To test this hypothesis we compared the kinetics of locomotion in two mammals of body size close to those of early mammals (< 40 g), both with parasagittally oriented limbs: a crouching shrew (Blarina brevicauda; 5 animals, 17 trials) and a more upright vole (Microtus pennsylvanicus; 4 animals, 22 trials). As predicted, voles used less mechanical power per unit body mass to perform steady locomotion than shrews did (P = 0.03). However, whi...

哺乳动物早期演化的一项重要趋势，是从四肢呈外展状态的匍匐姿态（sprawling stance），转变为四肢更垂直向下的矢状面姿态（parasagittal stance）。在完成这一转变后，多数哺乳动物又从蹲伏姿态进一步演化为更为直立的姿态。有假说认为，这些姿态转变的结果之一，是运动所需的总机械功率有所降低——这是因为身体的侧向加速度随之减小，而肢体朝向变化带来的能量消耗也因此降低。为验证这一假说，我们对两种体型接近早期哺乳动物（体重＜40克）、且四肢均呈矢状面定向的哺乳动物开展了运动动力学对比分析：一种是蹲伏姿态的短尾鼩鼱（Blarina brevicauda；共5只个体，17组试验），另一种是姿态更直立的草原田鼠（Microtus pennsylvanicus；共4只个体，22组试验）。正如所预测的那样，在进行稳态运动时，田鼠单位体重所需的机械功率要低于鼩鼱（P = 0.03）。然而，whi...