Meeting Abstract
Across vertebrate diversity, limb bone morphology is typically expected to reflect differences in the habitats and functional tasks with which species contend. Arboreal vertebrates are often recognized to have longer limbs than terrestrial relatives, a feature thought to help extend the reach of limbs across gaps between branches. Among terrestrial vertebrates, longer limbs can experience greater bending moments that might expose bones to a greater risk of failure. However, changes in habitat or behavior can impose changes in the forces that bones experience. If locomotion imposed lower loads in trees than on the ground, such a release from loading demands might have produced conditions under which potential constraints on the evolution of long limbs were removed, making it easier for them to evolve in arboreal species. We tested for such environmental differences in limb bone loading using the green iguana (Iguana iguana), a species that readily walks over ground and climbs trees. We implanted strain gauges on the femur and compared loads between a level-stiff surface, a level-compliant surface, and an inclined (60 deg) surface, with the latter two treatments modeling substrate conditions of tree branches and trunks, respectively. Shear strains were similar across treatments; however, counter to expectations, bending strains were greater for both compliant and inclined surfaces than for stiff, level ground. These results suggest that evolutionary changes in limb length among arboreal species may have occurred despite increases in limb bone loads during movement through the trees. The advantages of longer limbs for arboreal taxa may, therefore, have outweighed potential costs.
