Meeting Abstract
The morphology of vertebrate limb bones is generally expected to reflect differences in the functional demands that they experience. Such different demands often arise through changes in habitat. For example, the limbs of arboreal vertebrates are often longer than those of terrestrial relatives, a feature that may improve reach across gaps between branches, but which would be expected to incur a higher risk of bending and breakage during locomotion on the ground. In addition to resulting from changes in habitat, different demands might also arise between the fore- and hindlimbs, which themselves might be affected differently by changes in habitat. To test this hypothesis, we compared previous measurements of in vivo strains from the femur of green iguanas during simulated climbing to new measurements of bone strain from the humerus. Trials were conducted for inclined climbing, and walking on a level, compliant surface, to test whether loads under these conditions were lower than those on stiff, level ground. Such a pattern would suggest that “biomechanical release” from loading demands may have facilitated the evolution of longer limbs. We found that both inclined and compliant conditions increased femoral strains when compared to standard level conditions. However, unlike the hindlimb, there was not a consistent pattern of lower or higher bone strains for the forelimb during trials that simulated arboreal conditions. Synthesizing results from the fore- and hindlimbs, biomechanical release seems to be an unlikely mechanism that promoted limb elongation in arboreal taxa. Instead, limb bone adaptations in arboreal habitats seem to have been driven by selective pressures other than their response to loading.
