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