Hoffmann's two‐toed sloth I: Complete myology in the thoracic limb of <i>Choloepus</i> (Pilosa: Xenarthra)

Two- and three-toed sloths are sympatric throughout the neo-tropical rainforests of the Americas. Despite numerous morphological and physiological traits that are convergent, each genus has distinctive ecological and behavioral preferences, including a higher frequency of suspensory walking and posture in two-toed forms. Moreover, complete limb muscle myology is available for only a single species of two-toed sloth, whereas muscle architectural properties are unknown for the genus Choloepus. Part one of this study provides novel descriptions and detailed muscle maps of muscular arrangements for the entire forelimb of Hoffmann's two-toed sloth (C. hoffmanni). Origin, insertion, action, and fiber orientation were documented in 59 forelimb muscles. Several notable features in Choloepus are observed and discussed for their functional implications including: (1) rearrangements to the origin of m. trapezius pars cervicalis and m. biceps brachii short head; (2) a shoulder-elbow muscle chain; (3) extra bellies of the mm. rhomboidei, m. pectorales superficialis, and m. flexor digitorum profundus compared with those same muscles in Bradypus. The observed complexity of muscle traits is suggested to be consistent with the notion of selection for frequent and prolonged suspension (hanging and walking) in two-toed sloths that diverge from more simplified muscular arrangements observed in three-toed sloths, which exhibit a lower frequency of suspensory support and posture. Functional capacities of individual muscles and groups will be determined through quantifications of force, torque, and power in part two of this study to further test this hypothesis.