Object sorting by chimpanzees and monkeys.

Chimpanzees and two species of monkey were compared on sorting tasks. When the sets of objects differed in multiple ways, most chimpanzees learnt to sort whereas only a few monkeys performed better than chance (and then inconsistently). When the objects within a set differed in one or two respects, so that sorting by one principle entailed "unsorting" by one or more alternative principles, chimpanzees had greater difficulty. Two young animals did rapidly learn to sort by color and size ("unsorting" by size and shape or by color and shape). They did not sort pictorial material and had difficulty in sorting by shape if "unsorting" by both color and size. The sorting task differentiated between the cognitive capacities of chimpanzees and monkeys whereas other tasks did not.