In vivo locomotor strain in the hindlimb bones of Alligator mississippiensis and Iguana iguana: Implications for the evolution of limb bone safety factor and non-sprawling limb posture

Limb postures of terrestrial tetrapods span a continuum from sprawling to f ully upright; however, most experimental investigations of locomotor mechan ics have focused on mammals and ground-dwelling birds that employ parasagit tal limb kinematics, leaving much of the diversity of tetrapod locomotor me chanics unexplored. This study reports measurements of in vivo locomotor st rain from the limb bones of lizard (Iguana iguana) and crocodilian (Alligat or mississippiensis) species, animals from previously unsampled phylogeneti c lineages with non-parasagittal limb posture and kinematics, Principal str ain orientations and shear strain magnitudes indicate that the limb bones o f these species experience considerable torsion during locomotion. This con trasts with patterns commonly observed in mammals, but matches predictions from kinematic observations of axial rotation in lizard and crocodilian lim bs. Comparisons of locomotor load magnitudes with the mechanical properties of limb bones in Alligator and Iguana indicate that limb bone safety facto rs in bending for these species range from 5.5 to 10.8, as much as twice as high as safety factors previously calculated for mammals and birds. Limb b one safety factors in shear (3.9-5.4) for Alligator and Iguana are also mod erately higher than safety factors to yield in bending for birds and mammal s. Finally, correlations between limb posture and strain magnitudes in Alli gator show that at some recording locations limb bone strains can increase during upright locomotion, in contrast to expectations based on size-correl ated changes in posture among mammals that limb bone strains should decreas e with the use of an upright posture. These data suggest that, in some line ages, strain magnitudes may not have been maintained at constant levels thr ough the evolution of a non-sprawling posture unless the postural change wa s accompanied by a shift to parasagittal kinematics or by an evolutionary d ecrease in body size.