A major challenge for integrative studies of primate locomotor evolution is elucidating the structural basis for the energetic cost of locomotion, as metabolism is thought to have played a significant role in human and nonhuman primate evolution. The main goal of this thesis was to link mechanics, anatomy and locomotor cost during walking and running gaits in a nonhuman primate, Lemur catta. To do this, I carried out a series of four experiments collecting a combination of O₂, video, force platform and anatomical data. The main results of these experiments were:​ (1) Lemur catta use 'passive' mechanisms (i.e. pendulums and springs) during walking and running in order to minimize the external mechanical work required to lift and accelerate the center of mass. (2) Most of the increase in the cost of locomotion with speed is predicted by the change in the rate of muscle force production (1/t_c). I also demonstrated that lemurs selfselect walking speeds that minimize their locomotor costs. (3) The increase in the cost of locomotion unaccounted for by the faster rate of muscle force production is explained as an increase in the magnitude of limb muscle force generated per Newton of ground force, applied to support and maintain forward progression of the center of mass. The change in muscle volume from a walk to a run being driven by a decrease in mechanical advantage and, to a much smaller degree, the activation of longer, parallel-fibered muscles. This is consistent with the view of limb function as strut-like in walking and spring-like in running gaits. (4) The cost of locomotion is significantly increased with added trunk mass of ~16% and -26%, but with no significant change in the effectiveness of the passive mechanisms for walking and running. Taken together, this thesis links anatomical structure, mechanics and energetics of terrestrial locomotion in primates, and furthers our understanding of the musculoskeletal basis of locomotor cost in other mammals. This work is also useful for estimating walking and running costs in nonhuman primates in the wild as well as reconstructing the cost of locomotion and cost of transport in fossil taxa.