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Spinozzi, G., Natale, F., Langer, J., & Brakke, K. E. (1999). Spontaneous class grouping behavior by bonobos (Pan paniscus) and common chimpanzees (P. troglodytes). Anim. Cogn., 2(3), 157–170.
Abstract: Two experiments investigated spontaneous class grouping behavior by human-enculturated and language-reared bonobos (Pan paniscus) and common chimpanzees (P. troglodytes). In experiment 1, three chimpanzees ranging in age from 6 to 18 years were presented with six objects. The objects embodied three conditions: additive, multiplicative and disjoint classes. All chimpanzees spontaneously produced single- and two-category classifying. In experiment 2, six chimpanzees ranging in age from 6 to 21 years were presented with 12 objects in the same class conditions. Chimpanzees mainly produced single-category classifying. Their two-category classifying was more rudimentary than that found in experiment 1. Chimpanzees did not produce any three-category classifying which would be necessary to construct the hierarchies that humans begin to construct during early childhood.
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