Insufficient visual information leads to spontaneous bipedal walking in Japanese monkeys

Japanese monkeys walked spontaneously on their hind limbs, when their vision was impaired either by narrowing the visual field or by reducing the incoming light. These variables were manipulated via goggles with translucent pipes and neutral density filters. The bipedal locomotion was observed more frequently as the impairment of the incoming visual information increased. It is very likely that facultative bipeds walk on their hind limbs when they feel the need to "free" their forelimbs to grope their way.