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The authors investigated choice behavior by chimpanzees in five experiments involving choices between different amounts of food. Chimpanzees did not maximize the amount of food they obtained when choosing between a single 20-g banana piece and another option containing a 20-g piece and a 5-g piece. This was true even though they successfully discriminated between 20-g and 25-g banana pieces in other trials. When items in the mixed option were stacked, however, the chimpanzees chose the larger amount. Later experiments indicated that changing the magnitude of the two amounts did not change performance if the difference in magnitude between the two options remained the same (e.g., 40 g plus 10 g vs. 40 g). However, chimpanzees did improve when the two-item option was increased in its magnitude relative to the single slice (e.g., 20 g plus 10 g vs. 20 g). These results indicated that chimpanzees undervalued the total amount of food in sets when items differed in size and did not appear to be whole. Another experiment confirmed that it was this notion of wholeness that evoked suboptimal responding because chimpanzees were successful in the same comparisons with a different type of food that appeared less fractionated when presented as two pieces. These results provide evidence of suboptimal responding in some natural choice situations that prevents chimpanzees from maximizing food intake.

Perception of food amounts by chimpanzees (Pan troglodytes): The role of magnitude, contiguity, and wholeness.