Natural category discrimination in chimpanzees (Pan troglodytes) at three levels of abstraction

Two adult chimpanzees were presented with a series of natural category discrimination tasks on a touch screen computer, in which the discriminations varied in degree of abstraction. At the concrete level, discriminations could be made on the basis of single perceptual features, but at the more abstract level, categories were more inclusive, containing exemplars with variant perceptual features. For instance, at the most abstract level, the chimpanzees were required to select images of animals rather than nonanimals, and exemplars within both categories were perceptually diverse. One chimpanzee showed positive transfer at each level of abstraction but required more sessions to reach criterion as the discriminations became more abstract. The other chimpanzee failed to demonstrate consistent significant acquisition of a concept. The results indicate that unlike other apes and black bears, tested previously, chimpanzees found the most abstract discriminations the most difficult to acquire. Analyses of the features of pictures that yielded high or low accuracy revealed no significant differences on several key features, suggesting that the presence of facial features, eyes, or specific coloration did not control responding. In addition, the chimpanzees performed more accurately with photos judged as less typical exemplars of the category by human raters. However, responses to pictures of particular species suggest that chimpanzees may rely on perceptual similarity to familiar exemplars when acquiring experimenter-defined natural categories.