Meeting Abstract

57.4  Thursday, Jan. 6  Two and Four-legged Locomotion in Capuchin Monkeys: Implications for the Evolution of Human Bipedalism. DEMES, B*; O’NEILL, MC; Stony Brook Univ.; Stony Brook Univ. bdemes@ms.cc.sunysb.edu

Capuchin monkeys are known to locomote on both four and two legs in the wild, adopting a bipedal gait when transporting food items and tools. Understanding how their locomotor mechanics differ between quadrupedalism and bipedalism may provide insight into the origins of hominin locomotion. We present 3D video and force plate measurements of capuchins (n=5) locomoting on two and four legs overground across a broad speed range. Our animals used running walks (i.e., single-foot or ambles) and galloping gaits on four legs, and a slow running gait on two legs. Center of mass (COM) energy exchange is <20% in bipedalism, and COM work and power are 4 to 5 times higher than in quadrupedalism at equivalent speed. The COM data are consistent with limb joint kinematics, which indicate that capuchins use a ‘Groucho run’ on two limbs. That is, their limb posture is bent-hip, bent-knee, hip height decreases during stance phase, and faster strides have short aerial phases (duty factors = 0.65 – 0.44). The bipedal speed range is narrow and maximum speed much lower than in quadrupedalism. Our results demonstrate that facultative bipedalism need not be limited to walking. They reinforce the notion of morphological constraints on human-like upright striding bipedalism in nonhuman primates. They indicate that non-human primates using a bent-hip, bent-knee bipedal gait increase their positive COM work relative to four-legged locomotion. Taken together, our data suggest that transitioning to bipedalism is associated with increased COM work and decreased overground speed. Therefore, we suggest that two-legged locomotion in the earliest hominins was unlikely to have been motivated by these factors. Supported by NSF BCS 0548892 (BD).