Kinematics of bipedal locomotion while carrying a load in the arms in bearded capuchin monkeys (Sapajus libidinosus).

Understanding the selective pressures that drove the evolution of bipedalism in the human lineage may help inform researchers about the locomotor mode(s) of pre-hominin ancestors. Several selective pressures have been hypothesized, including the need to carry food, tools, or infants. Bearded capuchin monkeys are an excellent primate in which to examine the hypothesis that carrying supported the evolution of bipedalism because they are morphologically generalized and in some ways similar to Miocene hominoids, from which the transitional biped evolved. Additionally, bearded capuchins regularly move bipedally while carrying tools that represent a significant portion of their body mass. Here, we examined the spatio-temporal and kinematic gait parameters in a wild setting of Sapajus libidinosus moving bipedally while carrying a stone tool, as well as unloaded bipedal tufted capuchins in the lab. Results indicate that compared with humans, the monkeys move with a more bent-hip, bent-knee posture during both types of bipedalism, as expected. Few differences exist in spatio-temporal or kinematic parameters within species across load-carrying and unloaded bipedalism. The capuchin ankle, however, during load-carrying goes through a greater range of motion in relatively less time than both humans and unloaded capuchins. Data from this study provide the first quantitative data on bipedalism during load-carrying by wild primates in a natural setting. As such, they are a useful comparative reference for understanding bipedalism, particularly during load-carrying.