Meeting Abstract
In many vertebrate lineages that have reinvaded aquatic habitats, major morphological changes have evolved in the forelimb, including shifts from tubular limbs to elongate flippers. In contrast, changes to the hind limb have often involved limb reduction as well as flattening. How might changes in limb loading between environments have facilitated such changes? In vivo strain data from the hind limbs of semi-aquatic turtles have indicated that femoral strain magnitudes in water were only one fourth of those on land, and that torsion was particularly reduced during swimming. Given the retention of prominent forelimbs in most secondarily aquatic species, we predicted smaller differences between loads in water and on land for the forelimb, and tested this prediction by comparing strains on the humerus of a semi-aquatic turtle species (Pseudemys concinna) during aquatic swimming and terrestrial walking. In contrast to these predictions, aquatic bending strains on the humerus show a similar reduction, to levels roughly one fourth of those on the femur. Shear strains are also lower in water for the humerus; however, unlike the femur, this appears to relate to a reduction in overall strain magnitudes, rather than a reduction in limb twisting and the orientation of loads. These results indicate that similar changes in loading may occur between the forelimb and hind limb in swimming versus walking, but that they may occur through different mechanisms that relate to differences in the functional roles of these structures.
