The serratus anterior pars caudalis muscle of nonhuman primates displays anatomical differences among genera that can be attributed to differences in the mechanical demands placed on these genera by their diverse locomotor: behaviors. In primates that engage extensively in climbing and suspensory behaviors, the caudal digitations of this fan-shaped muscle are aligned more nearly parallel to the long axis of the trunk. In order to clarify the selective factors promoting such a morphological change, we have conducted a telemetered electromyographic study of the caudal and middle digitations of the serratus anterior pars caudalis. During voluntary elevation of the forelimb, only the middle, more obliquely disposed digitations are powerfully recruited. The caudal digitations are either inactive or function just to initiate scapular rotation. During locomotion, the middle digitations act in the swing (recovery) phase, whereas the caudal digitations are predominatly active in the support (propulsive)These Pashe findings suggest that the caudal digitations are important in propelling the trunk past the scapula during locomotion. Evolution of a fiber orientation more parallel to the long axis of the trunk is suggested to have occurred in broad chested primates for the purpose of facilitating locomotor behaviors requiring caudal scapular retraction for propulsion, but which would be deleteriously affected if such retraction were linked to simultaneous ventral displacement of the shoulder girdle. In its current state, the human serratus anterior seems clearly adapted for arm-raising functions and indicates descent from a small ape with a thoracic shape similar to atelines.