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This long-term study of woolly monkey (Lagothrix) locomotor and postural behaviour employs methods identical to those used during a previous study of the locomotion and posture of two species of Ateles, allowing a detailed comparison between the two genera, which are strong competitors in extensive parts of the Amazon basin and northern Andes. As in Ateles, Lagothrix locomotion can be divided into five patterns, based on limb usage: quadrupedal walking and running, suspensory locomotion, climbing, bipedalism (very rare in wild woolly monkeys) and leaping. Lagothrix differs from Ateles primarily in its greater reliance on quadrupedal locomotion during both travel and feeding and on its de-emphasis of the use of suspensory locomotion as compared to Ateles, while the use of climbing and leaping is roughly equal in the two genera. Lagothrix exhibits more generalised (primitive) locomotive behaviour in accordance with its morphology, in comparison to the more specialised Ateles. The generic differences reflect differences in habitat use and particularly foraging ecology.

The anatomy of the wrist joint is described in specimens of Ateles geoffroyi, Ateles sp., Lagothrix lagotricha, Pongo pygmaeus and Gorilla gorilla gorilla. It is demonstrated that the semibrachiating monkeys retain a primitive quadrupedal type of articulation with the ulnar styloid process contacting both pisiform and triquetral; the fundamentally different structure of this articulation in Pongidae is confirmed. The implications of these findings for the interpretation of hominoid evolution are briefly discussed.