The

acquisition of a tool-using ability was investigated in six young chimpanzees (Pan troglodytes, 2 to 4 years old). Age-matched pairs were presented with a horizontal transparent tube with a food item inserted in the center, and a wooden tool. Insertion of the tool into the tube was required in order to obtain the food item. One of each pair was exposed to a model performing the task successfully,whereas the age-matched peer was not. Following acquisition, subjects were tested with more complex versions of the task to evaluate their comprehension. Age affected acquisition; older individuals learned to solve the task in fewer number of trials than younger chimpanzees. The presence of a model influenced acquisition only in the 3-and-4 year-old groups and not in the 2-year-old group. Moreover,older individuals made fewer errors when faced with tools requiring modification, and the performance of older individuals on these complex tasks improved with limited practice. These results are related to recent findings on cognitive development in chimpanzees indicating that self-recognition emerges between 24 and 30 months and that 4 year-old chimpanzees can imitate novel arbitrary actions. Comparisons with human cognitive developmental data and findings on the same task with older apes point to the link between the emergence of imitation, self recognition, and comprehension of the cause-effect relation present in this task. Competence in all these domains is somewhat delayed in chimpanzees compared to humans.