Three chimpanzees with a history of conditional and numeric token training spontaneously matched relations between relations under conditions of nondifferential reinforcement. Heretofore, this conceptual ability was demonstrated only in language-trained chimpanzees. The performance levels of the languagenaive animals in this study, however, were equivalent to those of a 4th animal-Sarah-whose history included language training and analogical problem solving. There was no evidence that associative factors mediated successful performance in any of the animals. Prior claims of a profound disparity between language-trained and language-naive chimpanzees apparently can be attributed to prior experience with arbitrary tokens consistently associated with abstract relations and not language per se. Early reports by some investigators indicated that language-trained chimpanzees differed profoundly in their problem-solving skills from chimpanzees who had not had such training (D. Premack, 1983a, 1983b, 1984, 1986; Savage-Rumbaugh, 1986). Language-trained chimpanzees solved conceptual problems that language-naive chimpanzees failed. D. Premack's language-trained animals, for example, reportedly were superior to the language-naïve animals on problems that required reasoning about relations between relations but that could not be solved merely on the basis of correspondence of physical features. The discrepancy between language-trained and language-naïve chimpanzees reported by D. Premack was based originally on the performances of seven African-born chimpanzees raised from infancy by human caregivers; three of the seven had received language training and four had not. Premack argued that the disparity in performance between these two groups of animals on such tasks could not be attributed to either age differences or their relative test sophistication (cf. D. Premack, 1984). Rather this profound disparity could be attributed only to whether or not an animal had received language training as originally described by D. Premack (1976). Short strings of rule-governed plastic words formed the physical basis of this language, which the chimpanzees used in both comprehension and production. See

Studies of the conceptual abilities of nonhuman primates demonstrate the substantial range of these abilities as well as their limitations. Such abilities range from categorization on the basis of shared physical attributes, associative relations and functions to abstract concepts as reflected in analogical reasoning about relations between relations. The pattern of results from these studies point to a fundamental distinction between monkeys and apes in both their implicit and explicit conceptual capacities. Monkeys, but not apes, might be best regarded as "paleo-logicans" in the sense that they form common class concepts of identity on the basis of identical predicates (i.e., shared features). The discrimination of presumably more abstract relations commonly involves relatively simple procedural strategies mediated by associative processes likely shared by all mammals. There is no evidence that monkeys can perceive, let alone judge, relations-between-relations. This analogical conceptual capacity is found only in chimpanzees and humans. Interestingly, the "analogical ape," like the child, can make its analogical knowledge explicit only if it is first provided with a symbol system by which propositional representations can be encoded and manipulated

Generally speaking, the study of concepts in cognitive psychology is anthropocentric with respect to both content and theory. A broader comparative perspective on the various forms of concept learning not only provides a more inclusive view of conceptual behavior, but it also provides a more objective perspective from which to identify underlying processes. We suggest that several of the major varieties of conceptual classes claimed to be uniquely human are also exhibited by nonhuman animals. We present evidence for the formation of several sorts of conceptual stimulus classes by nonhuman animals: perceptual classes involving classification according to the shared attributes of objects, associative classes or functional equivalences in which stimuli form a class based on common associations, relational classes, in which the conceptual relationship between or among stimuli defines the class, and relations between relations, in which the conceptual (analogical) relationship is defined by the relation between classes of stimuli. We conclude that not only are nonhuman animals capable of acquiring a wide variety of concepts, but that the underlying processes that determine concept learning are also likely to be quite similar.

Four infant chimpanzees learned a matching-to-sample task when only two training stimuli were used. They then spontaneously transferred the matching concept to novel items, including three-dimensional objects and fabric swatches, without any experimenter-provided differential feedback. These results support the view that the matching concept is broadly construed by chimpanzees from the beginning and does not depend upon explicit training.

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