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 if 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. 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 shear strains show similar reductions between swimming and walking for forelimb and hindlimb, but these reductions are produced through different mechanisms.

Youngetal_TurtleHumerusStrainData_DryadHumeral strain data for Pseudemys concinna during walking and swimming trials.