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.