Anatomical, physiological, and kinesiological study of bipedal capability in the Japanese macaque

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Habitual bipedalism is only observed in humans among living primates whereas living humans are highly specialized bipeds. This fact causes a lack of comparative material to study adaptive process of human bipedalism. To cope with this problem, we conducted a morphological, physiological and kinesiological study on bipedally trained macaques for traditional money performance. Experiments were conducted with the Suo Monkey Performance Association (Kumamoto) and investigated tocomotor energetics and kinematics during locomotion on a treadmill. We examined locomotor energetics on two subjects (10 and 4 yrs in age). Energy expenditure was estimated as the rate of CO2 production standardized by the body mass. The walking velocity was 1.5-4.5 km/h. Energetic costs increased linearly correlated with walking velocities in bipedal and quadrupedal walk. Cost of bipedal walk was greater than that of quadrupedalism by ca, 30% in the elder subject and by 20-25% in the younger subject These ratios are rather constant in each individual regardless of walking velocity. We tentatively converted the observed CO2 production rate to oxygen consumption using a (empirically known) normal respiratory quotient (0.85) and obtained similar values revealed in previous studies. Kinematic analysis by using a two video camera system revealed that the trained monkeys took longer strides and fewer stride frequencies compared with normal (experimental) monkeys. These features are correlated with larger joint angles of the hindlimb and the knee joint of the trained monkey exhibited a human-like double knee action. This causes an upward translation of the trunk during the single limb support phase, which is observed in the double limb support phase in the normal monkeys. These kinematic features suggest that these trained macaques saved the costs of bipedalism by developing more human like bipedal gait, indicating the potential and plasticity for bipedalism.