In addition to their knowledge about the social world, primates must also understand how to identify and categorize objects in the physical world. Discovering appropriate categories is a challenge, however, because primates could sort objects in an infinite number of possible ways. Today, I will discuss how primates successfully classify objects in one physical domain: the domain of food. All organisms must quickly learn to find things to eat, yet we still know relatively little about how primates distinguish food items from non-food items. I tested the abilities of semi-free-ranging rhesus monkeys (Macaca mulatta) to rapidly learn that a novel object was edible, and to generalize their learning to other objects, in a spontaneous choice task. Adult monkeys watched as a human experimenter first pretended to eat one of two novel objects and then placed replicas of the objects at different locations. Subjects selectively approached the previously eaten object. I then tested how monkeys generalized their preference for an edible object by first allowing them to watch a human experimenter eat one of two objects and then present subjects with two new objects composed of the same substance but differing from the original, edible object in shape or color. Monkeys ignored changes in the shape of the object and generalized from one edible object to another on the basis of color in conjunction with other substance properties. These results were then replicated with two new paradigms: a searching time method and an expectancy violation experiment, in which subjects also generalized to new food exemplars using the feature of color. Finally, we extended this work to infant rhesus monkeys and found that, like adults, they too used color to generalize to novel food objects. In contrast to adults, however, infants extended this pattern of generalization to non-food objects (i.e.,, tools). These results suggest that infants form broader object categories than adults, and that food categories become sharpened as a function of maturational or experiential factors. These results are compared with similar results on the rhesus monkey's understanding of the functionally relevant features of tools.

SANTOS, Laurie R.
Department of Psychology,
Harvard University
William James Hall
33 Kirkland St., Rm. 970
Cambridge, MA 02138, USA
lrsantos@wjh.harvard.edu