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This review surveys some illustrative experiments on categorization of visual stimuli by animals, preferably pigeons (Columba livia). Traditionally, it has been assumed that the ability to categorize stimuli and to extend the classification to novel members of the categories involves conceptualization. In the past, however, pigeon studies suffered from overly simplistic assumptions concerning the perceptual aspects of natural categorization. Recent evidence suggests that the way in which pigeons sort natural categories does not require conceptual abilities, i.e., learning that transcends pictorial memory or learning to attend to the class-characteristic features. We found that pigeons classify visually complex, natural images (male and female human faces) by means of their global properties, which covaried with the semantic content of the categories. The hypothesis proposed here is that natural categories and visual classes are coextensive, i.e., that behavioral and perceptual contingencies are conjointly correlated with environmental dimensions of variance. Hence, the pigeon's ability of open-ended categorization may result from the generic nature of natural categories and natural selection that has equipped animals with considerable adaptations for dealing with the categorization problem in this very sense.