Acquisition of an externalized eye by Japanese monkeys.

Many animals use tools to augment motor function ("motor tools", like rake), while the use of tools to acquire sensory information or to augment sensory function ("sensory tools", like endoscope) has been reported only in humans. In the present study, we trained Japanese monkeys to acquire the sensory tool use to re-construct a possible developmental course of the human-specific tool use via motor tool use training. After they mastered the rake use, we systematically introduced a series of external mirror and video arrangements, so as to separate visual cues from their actual origins in visuomotor space. Finally, the monkeys could acquire the use of sensory tool-a sort of endoscope attached to a rake-to explore the experimental space to find and retrieve the food. The results indicated a critical role of environmental control to develop even higher order behavioral sequences like human-specific sensory tool use in nonhuman primates.