Three-Dimensional Limb Kinematics in Brown-Throated Three-Toed Sloths (Bradypus variegatus) During Suspensory Quadrupedal Locomotion.

Suspensory locomotion differs significantly from upright quadrupedal locomotion in mammals. Nevertheless, we know little concerning joint kinematics of suspensory movement. Here, we report three-dimensional kinematic data during locomotion in brown-throated three-toed sloths (Bradypus variegatus). Individuals were recorded with four calibrated high-speed cameras while performing below-branch locomotion on a simulated branch. The elbow (range 73°-177°; mean 114°) and knee (range 107°-175°; mean 140°) were extended throughout support phase, with elbow extension increasing with speed. Both the fore- and hindlimb displayed abducted proximal limb elements (i.e., arm and thigh) and adducted distal elements (i.e., forearm and leg) during all support phase points. Comparisons of elbow and knee angles between brown-throated three-toed sloths and Linnaeus's two-toed sloths (Choloepus didactylus) showed that brown-throated three-toed sloths had significantly more extended joint positions during all support phase points. Additionally, across all kinematic measurements, brown-throated three-toed sloths showed significant differences between homologous fore- and hindlimb segments, with the knee being more extended than the elbow and the arm being more abducted than the thigh. These results are consistent with previously established morphological and behavioral differences between extant sloth genera, with three-toed sloths showing significantly longer forelimbs than hindlimbs and typically favoring locomotion on angled supports. Our findings show that, despite overall similarities in the use of below-branch quadrupedal locomotion, the two sloth lineages achieve this locomotor mode with differing kinematic strategies (e.g., degree of joint flexion). These differences may be attributed to the distinct evolutionary pathways through which obligate suspensory locomotion arose in each lineage.