Levels of abstraction in orangutan (Pongo abelii) categorization.

Levels of abstraction have rarely been manipulated in studies of natural concept formation in nonhumans. Isolated examples have indicated that animals, relative to humans, may learn concepts at varying levels of abstraction with differential ease. The ability of 6 orangutans (Pongo abelii) of various ages to make natural concept discriminations at 3 levels of abstraction was therefore investigated. The orangutans were rewarded for selecting photos of orangutans instead of humans and other primates (concrete level), primates instead of other animals (intermediate level), and animals instead of nonanimals (abstract level) in a 2-choice touch screen procedure. The results suggest that, like a gorilla (Gorilla gorilla gorilla) tested previously (Vonk & MacDonald, 2002), orangutans can learn concepts at each level of abstraction, and unlike other nonhumans, most of these subjects rapidly learned the intermediate level discrimination.