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J Comp Psychol. 2001 Dec;115(4):385-91.

Spontaneous use of magnitude discrimination and ordination by the orangutan (Pongo pygmaeus).

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National Zoological Park, Smithonian Institution, Washington, DC 20008, USA.


The ability to discriminate quantity is descriptive of general cognitive ability. In this study, the authors presented 2 orangutans (Pongo pygmaeus) with a quantity judgment task. Each trial consisted of 2 choices, ranging from 1 to 6 food items in each. The orangutan chose 1 of the quantities, which was removed, and the remaining array was given as a reward. In contrast to chimpanzees previously tested on the same task (S. T. Boysen & G. G. Berntson, 1995; S. T. Boysen, G. G. Berntson, M. B. Hannan, & J. T. Cacioppo, 1996; S. T. Boysen, K. L. Mukobi, & G. G. Berntson, 1999), the orangutans optimized their performance. Orangutans, therefore, attend to differences in magnitude and can spontaneously use ordinality. Results also suggest a cognitive difference between chimpanzees and orangutans.

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