Forelimb and hind limb loading patterns during below branch quadrupedal locomotion in the two‐toed sloth

Abstract

AbstractThe living sloths are the most suspensory of all extant mammals, and therefore represent ideal models for investigating the effects that suspensory behaviours have on bone and joint morphology. While the anatomy and kinematics of sloths are well known, no research has reported kinetic patterns of sloth locomotion. This study examines peak force patterns generated by the two‐toed sloth Choloepus didactylus during below branch quadrupedal walking to infer how kinetic patterns of sloths compare to data reported on non‐human primates. Values for vertical, fore‐aft, and mediolateral peak forces were collected for the forelimb and hind limb, and analyses between the magnitude and timing of these peaks were compared between forelimbs and hind limbs. Patterns and timing of fore‐aft peak forces were similar between sloths and non‐human primates, and were characterized by first a propulsive force as the limb first made contact with the support followed by braking force prior to lift‐off. Similarly, both sloths and primates demonstrate a medially directed force bias onto the substrate during below branch quadrupedal locomotion, although the magnitude observed in sloths exceeds values reported in primates. Peak vertical forces applied by the forelimbs and hind limbs of sloths were not statistically different in magnitude from each other. Data from this study indicate the forelimbs and hind limbs of sloths are functioning similarly to each other during below branch quadrupedal locomotion, and that forelimb‐biased weight support, the pattern typical of primates, is not a mechanical requirement of suspensory locomotion across all mammals. These findings provide important information about the mechanical necessities of below branch movement, and data from this study should be used when considering mechanical convergence among suspensory taxa and the interpretation of suspensory limb adaptations in the fossil record.