A suspensory way of life: Integrating locomotion, postures, limb movements, and forces in two-toed sloths Choloepus didactylus (Megalonychidae, Folivora, Pilosa).

Over the last decade, we have learned much about the anatomy, evolutionary history, and biomechanics of the extant sloths. However, most of this study has involved studying sloths in controlled conditions, and few studies have explored how these animals are behaving in a naturalistic setting. In this study, we integrate positional activities in naturalistic conditions with kinematic and kinetic observations collected on a simulated runway to best capture the biomechanical behavior of Linnaeus's twotoed sloths. We confirm that the dominant positional behaviors consist of hanging below the support using a combination of forelimbs and hindlimbs, and walking quadrupedally below the branches. The majority of these behaviors occur on horizontal substrates that are approximately 5-10 cm in diameter. The kinematics of suspensory walking observed both in the naturalistic settings and on simulated arboreal runways are dominated by movement of the proximal limb elements, while distal limb elements tend to show little excursion. Joint kinematics are similar between the naturalistic setting and the simulated runway, but movements of the shoulder and hip tend to be exaggerated while moving in simulated conditions. Kinetic patterns of the two-toed sloth can be explained almost entirely by considering them as an inverted linked strut. However, medially directed forces toward the substrate were more frequent than expected in the forelimb, which may help sloths maintain a better "grip" on the substrate. This study serves as a model of how to gain a comprehensive understanding of the functional-adaptive profile of a particular species. [ABSTRACT FROM AUTHOR]