Data from: Humeral loads during swimming and walking in turtles: implications for morphological change during aquatic reinvasions

During evolutionary reinvasions of water by terrestrial vertebrates, ancestrally tubular limbs often flatten to form flippers. Differences in skeletal loading between land and water might have facilitated such changes. In turtles, femoral twisting is significantly lower during swimming than during walking, potentially allowing a release from loads that favor tubular shafts. However, flipper-like morphology in specialized tetrapod swimmers is most accentuated in the forelimbs. To test if the forelimbs of turtles are also released from torsional loading in water, we compared strains on the humerus of river cooters (Pseudemys concinna) between swimming and terrestrial walking. Humeral shear strains are also lower during swimming compared to terrestrial walking; however, this appears to relate to reduction in overall strain magnitudes, rather than a specific reduction in twisting. These results indicate that loads show similar changes between swimming and walking for the forelimb and hindlimb, but these changes are produced through different mechanisms.

在陆生脊椎动物演化重返水生环境的过程中，祖先型管状四肢常扁平化形成鳍肢（flippers）。陆地与水环境间的骨骼载荷（skeletal loading）差异，或推动了这类形态转变。针对龟类而言，其游泳过程中的股骨扭转载荷显著低于行走状态，这或有助于摆脱倾向于维持管状骨结构的载荷约束。不过，特化四足游泳动物的鳍肢状形态在前肢中表现得更为突出。为验证龟类前肢是否也能在水环境中摆脱扭转载荷，我们对比了锦龟（Pseudemys concinna）肱骨（humerus）在游泳与陆生行走时的应变水平。研究结果显示，游泳时肱骨的剪切应变（shear strains）同样低于陆生行走时，但这似乎源于整体应变幅度的降低，而非扭转载荷的特异性减弱。上述结果表明，前肢与后肢在游泳与行走状态下的载荷变化模式相似，但这类变化是通过不同机制实现的。