<b>Variations in humeral and femoral strains across body sizes and limb posture in American alligators</b>

Bone loading is a crucial factor that constrains locomotor capacities of terrestrial tetrapods. To date, limb bone strains and stresses have been studied across various animals, with a primary emphasis on constant bone loading in mammals of different sizes and variations in loading regimes across different clades and limb postures. However, the relationships between body size, limb posture, and limb bone loading remains unclear in animals with non-parasagittally moving limbs, limiting our understanding of the evolution of limb functions in tetrapods. To address this, we investigated <i>in vivo</i> strains of the humerus and femur in juvenile to subadult American alligators as they walked with various limb postures. We found that principal strains on the ventromedial cortex of the femoral midshaft increased with larger sizes among the three individuals displaying similar limb postures. This indicates that larger individuals experience greater limb bone strains when maintaining similar limb postures to smaller ones. Axial and shear strains in the humerus were generally reduced with a more upright limb posture, while trends in the femur varied among individuals. Given that larger alligators have been shown to adopt a more upright limb posture, the transition from sprawling to upright limb posture, particularly in the forelimb, might lead to or result from the evolution of larger body sizes in archosaurs. Moreover, both the humerus and femur experienced increased shearing compared to axial loads with a less upright limb posture, suggesting proportional changes in bone loading regimes throughout the evolution of limb posture.骨载荷是限制陆生四足动物运动能力的关键制约因素。迄今为止，学界已针对多种动物开展了肢骨应变与应力相关研究，核心聚焦于不同体型哺乳动物的恒定骨载荷，以及不同演化支、不同肢体姿势下的载荷模式差异。然而，对于肢体运动非矢状面的动物而言，其体型、肢体姿势与肢骨载荷之间的关联仍不明确，这限制了我们对四足动物肢体功能演化的认知。为此，本研究针对幼体至亚成体美洲短吻鳄开展实验，分析其在不同肢体姿势行走时的肱骨（humerus）与股骨（femur）的体内（in vivo）应变。我们发现，在三只肢体姿势相近的个体中，股骨中段腹内侧骨皮质的主应变随体型增大而升高。这表明，当体型更大的个体保持与小型个体相近的肢体姿势时，其肢骨承受的应变更高。肱骨的轴向与剪切应变通常随肢体姿势愈发直立而降低，而股骨的相关变化趋势则存在个体差异。已有研究表明体型更大的短吻鳄会采取更直立的肢体姿势，由此推测，从匍匐到直立的肢体姿势转变（尤其是前肢），可能推动了主龙类体型向更大方向演化，或是该演化过程的结果。此外，当肢体姿势不那么直立时，肱骨与股骨所承受的剪切载荷占比均高于轴向载荷，这表明在肢体姿势的演化过程中，骨载荷模式发生了相应的比例变化。