Acquisition and development of monkey tool-use: behavioral and kinematic analyses.

Four Japanese macaques were trained in the use of a T-shaped rake. Use the tool and development of the level of the skill of tool-use took place in three distinct stages. During stage 1, two of the monkeys seemed to use insight for initial solution, while fortuitous experiences led the other two monkeys to the solution. All the monkeys used the tool in a stereotyped manner and could retrieve food only when the tool was placed close to the food. At stage 2 the monkeys became able to manipulate the tool in various ways and became able to retrieve the food regardless of its position. By stage 3 they had developed the level of skill required for efficient retrieval. Further experiments revealed that the monkeys attempted to use unfamiliar objects which were similar to the original tool in shape, but not spherical or ring-shaped objects, to rake in the food.