Limb Bone Strains During Climbing in Green Iguanas: Testing Biomechanical Release as a Mechanism Promoting Morphological Transitions in Arboreal Vertebrates

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 humerus and femur, and then compared loads between treatments modeling substrate conditions of arboreal habitats. For hindlimbs, only surface angle indicated strain increases, whereas the forelimb lacked consistent evidence that treatments changed bone loading regimens directionally. In this system, biomechanical release seems to be an unlikely mechanism to have facilitated limb elongation; limb bone adaptations in arboreal habitats seem to be driven by selective pressures other than response to loading