Evidence for kind representations in the absence of language: experiments with rhesus monkeys (Macaca mulatta).

How do we come to recognize and represent different kinds of objects in the world? Some developmental psychologists have hypothesized that learning language plays a crucial role in this capacity. If this hypothesis were correct, then non-linguistic animals should lack the capacity to represent objects as kinds. Previous research with rhesus monkeys (Macaca mulatta) has shown that this species can successfully individuate different kinds of objects - monkeys who saw one kind of object hidden inside a box searched longer after finding a different kind of object. However, in these studies and the infant studies on which they were based, the objects to be individuated differed both in kind and in properties. Thus, subjects in these experiments may not be representing the kinds of objects per se, but instead only their immediate perceptual properties. Here, we show that rhesus monkeys successfully individuate different kinds of objects even when their perceptual properties are held constant. Although these data provide the best evidence to date that language is not necessary to represent kinds, we discuss our findings in terms of possible associative hypotheses as well.