Pump the brakes! The hindlimbs of three-toed sloths decelerate and support suspensory locomotion.

Modern tree sloths are one of few mammalian taxa for which quadrupedal suspension is obligatory. Sloth limb musculature is specialized for slow velocity, large force contractions that stabilize their body below branches and conserve energy during locomotion. However, it is unknown whether two- and three-toed sloths converge in their use of limb kinetics and if these patterns are comparable to how primates perform arboreal suspensory locomotion. This study addressed this need by collecting limb loading data in three-toed sloths (Bradypus variegatus; N=5) during suspensory walking. Sloths performed locomotor trials at their preferred speed on an instrumented beam apparatus with a force platform as the central supporting segment. Peak forces and impulses of the forelimb and hindlimb were recorded and analyzed in three dimensions. The hindlimbs of B. variegatus apply large braking forces greater in magnitude than peak forces generated by the forelimbs in propulsion, a pattern consistent with that observed in two-toed sloths. However, B. variegatus exhibits hindlimb-biased body weight support in vertical peak forces and impulse, with appreciable laterally directed forces in each limb pair, both of which vary from limb loading distributions in two-toed sloths. Moreover, body weight distribution between limb pairs is opposite to that employed by primates during quadrupedal suspension. Thus, there appear to be multiple strategies for achieving suspensory locomotion in arboreal mammals. These differences may be attributable to anatomical variation or phylogenetic position, but as of yet an explanation remains unknown. Future EMG analyses are expected to provide insight into how specific hindlimb muscle groups contribute to braking forces and stabilizing the center of mass of sloths during suspension.

Three toes and three modes: Dynamics of terrestrial, suspensory, and vertical locomotion in brown-throated three-toed sloths (Bradypodidae, Xenarthra).

Living sloths exhibit numerous anatomical specializations towards inverted quadrupedalism, however, previous studies have noted a more varied locomotor repertoire than previously anticipated. In this study, we present spatiotemporal gait characteristics and triaxial kinetic data from the brown-throated three-toed sloth (Bradypus variegatus) across three locomotor modes: terrestrial quadrupedal "crawling", suspensory walking, and vertical climbing. Compared to quadrupedal crawling and suspensory walking, B. variegatus adopted longer contact times and stride durations, larger duty factors, and greater speed during vertical climbing. Net fore-aft impulses were significantly greater during vertical climbing in both limb pairs than in quadrupedal crawling and suspensory walking. Functionally, during quadrupedal crawling and vertical climbing, both limb pairs served propulsive roles, while differentiation between a propulsive forelimb and braking hindlimb was observed during suspension. Net tangential forces differentiated vertical climbing kinetics from the other modes of locomotion, with the introduction of bidirectional pulling and pushing forces in the forelimb and hindlimb, respectively. The net mediolateral impulses were similar in vertical climbing and quadrupedal crawling as both limb pairs directed forces in one direction, whereas during suspensory walking, the laterally dominant forelimb was opposed by the medially dominant hindlimb. In total, this study provides novel data on the diverse locomotor dynamics in a slow-moving arboreal tetrapod and posits new testable hypotheses about the neuroplasticity and ease of transitioning between locomotor behaviors. The strikingly similar kinetic profiles of quadrupedal crawling and suspensory walking compared to vertical climbing suggest shared neuromuscular and mechanical demands between these mirrored locomotor modes.