The comparative and functional anatomy of appendicular musculature of Humboldt’s woolly monkey ( Lagothrix lagotricha ): What can a (mediocre) suspensory monkey tell us about and human locomotor evolution?

Similar to other members of the platyrrhine subfamily Atelinae, Humboldt’s wooly monkey ( Lagothrix lagotricha ) isa ‘tail‐assisted’ brachiator that more frequently engages in non‐suspensory forms of locomotion (i.e. quadrupedal climbing). As a group the atelines share a suite of derived forelimb skeletal characters that are specialized for brachiation and that are convergent with ‘richochetal’ brachiating apes ( Hylobates, Symphalangus ), quadrumanus suspensory apes ( Pongo ) and secondarily terrestrial apes retaining suspensory capabilities ( Pan, Gorilla ). The skeletal anatomy of lagothrix is similar to more suspensory atelines ( Ateles , Brachyteles ) and living apes, but is typically less derived for suspension and many skeletal characters are intermediate between highly suspensory and non‐suspensory anthropoids. While the functional morphology of the ateline skeleton is well documented, relatively little is known about the comparative and functional morphology of the forelimb and hindlimb musculature. Specifically, can locomotor differences between Lagothrix and more suspensory atelines and hylobatids be explained, in part, as a result of differences in muscletendon architecture? This study describes preliminary quantitative data on the muscle–tendon architecture (relative muscle mass and relative physiological cross‐sectional area or PCSA) of the fore‐ and hind‐limbs of Lagothrix lagotricha and Macaca sp. (n=9). Despite infrequent brachiation, Lagothrix forelimb relative muscle mass and PCSA is identical to similar results reported for hominoids. In contrast, the relative mass and PCSA of the hind‐limb musculature is most similar to quadrupedal monkeys (i.e. Macaca ). Specifically, Lagothrix resembles hylobatids in the concentration of muscle mass and force in the flexor compartments of the arm and forearm but is more similar to Macaca in similar measures associated with the extensor and flexor thigh and leg. This suggests that despite a reduced reliance on suspensory postures and brachiation, Lagothrix forelimb muscle‐tendon architecture may be, at least in part, a response to the minimum functional demands of brachiation but that similar selection pressures are not as prevalent in the hind‐limb.