Variations in humeral and femoral strains across body sizes and limb posture in American alligators

ABSTRACT Bone loading is a crucial factor that constrains locomotor capacities of terrestrial tetrapods. To date, limb bone strains and stresses have been studied across various animals, with a primary emphasis on consistent bone loading in mammals of different sizes and variations in loading regimes across different clades and limb postures. However, the relationships between body size, limb posture and limb bone loading remain unclear in animals with non-parasagittally moving limbs, limiting our understanding of the evolution of limb functions in tetrapods. To address this, we investigated in vivo strains of the humerus and femur in juvenile to subadult American alligators as they walked with various limb postures. We found that principal strains on the ventromedial cortex of the femoral midshaft increased with larger sizes among the three individuals displaying similar limb postures. This indicates that larger individuals experience greater limb bone strains when maintaining similar limb postures to smaller individuals. Axial and shear strains in the humerus were generally reduced with a more erect limb posture, while trends in the femur varied among individuals. Given that larger alligators have been shown to adopt a more erect limb posture, the transition from sprawling to erect limb posture, particularly in the forelimb, might be linked to the evolution of larger body sizes in archosaurs, potentially as a means to mitigate limb bone loading. Moreover, both the humerus and femur experienced decreased shear loads compared with axial loads with a more erect limb posture, suggesting proportional changes in bone loading regimes throughout the evolution of limb posture.