The comparative and functional anatomy of the forelimb muscle architecture of Humboldt's woolly monkey (Lagothrix lagotricha).

Humboldt's woolly monkey (Lagothrix lagortricha) is a ceboid primate that more frequently engages in plantigrade quadrupedalism (~89%) but is, like most other members of the subfamily Atelinae, capable of suspensory postures and "tail assisted" brachiation. That taxon's decreased reliance on suspension is reflected in the skeletal anatomy of the upper limb which is less derived relative to more frequently suspensory atelines (Ateles, Brachyteles) but is in many ways (i.e., phalangeal curvature, enlarged joint surfaces, elongated diaphyses) intermediate between highly suspensory and quadrupedal anthropoids. Although it has been suggested that muscle may have morphogenetic primacy with respect to bone this has not been explicitly tested. The present study employs analyses of Lagothrix upper limb muscle fiber length, relative physiological cross-sectional area and relative muscle mass to test whether muscular adaptations for suspensory postures and locomotion in Lagothrix precede adaptive refinements in the skeletal tissues or appear more gradually in conjunction with related skeletal adaptations. Results demonstrate that Lagothrix upper limb musculature is most like committed quadrupeds but that limited aspects of the relative distribution of segmental muscle mass may approach suspensory hylobatids consistent with only a limited adaptive response in musculature prior to bone. Results specific to the shoulder were inconclusive owing to under-representation of quadrupedal shoulder musculature and future work should be focused more specifically on the adaptive and functional morphology of the muscular anatomy and microstructure of the scapulothoracic joint complex.