Humeral loads during swimming and walking in turtles: implications for morphological change during aquatic...

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