Scaling of Gait, Limb Posture, Forces, and Inertial Properties in Terrestrial Locomotion of American Alligators Across a Thousand-Fold Increase in Body Mass

Abstract

Synopsis Scaling of body shape, gait, limb kinematics, and kinetics can inform about mechanical and energetic constraints influencing animal locomotion on land. However, our knowledge of terrestrial locomotion among tetrapods is biased toward animals that use parasagittal limb postures (mammals and birds). Whether species that use non-parasagittal limb postures (e.g., crocodylians and lizards) share similar scaling principles remains uncertain, especially toward the larger end of the body size spectrum. Here, we compare new data from large adult American alligators (140–223 kg) with previous data from smaller individuals (0.2–5.6 kg) to evaluate the scaling of gait, limb posture, ground reaction forces, and inertial properties of body segments over a size range spanning three orders of magnitude. Larger alligators walked with higher relative stride frequencies and shorter relative stride lengths at equal dimensionless speeds, suggesting that alligators prioritize stance phase mechanical work reduction at the expense of higher costs for swing as they increase in size. Limb posture also changed with body mass: ∼1 kg and larger alligators typically used more adducted and extended limbs, which is expected to mitigate limb muscle forces during stance. Ground reaction forces were increasingly hindlimb dominant as alligators grow to larger size, paralleled by a posterior shift in the center of mass. Counterintuitively, the forelimb grew faster in mass than the hindlimb, possibly because recruited muscle volume is closer to its maximum capacity in the forelimb, compelling larger alligators to keep up with increased mechanical demands via muscle growth more for the forelimb than the hindlimb. These results highlight the numerous changes in body proportions and locomotor performance that are required among animals that use non-parasagittal limb posture, as they grow from neonatal sizes into large adults.