When less is more: like humans, chimpanzees (Pan troglodytes) misperceive food amounts based on plate size

Abstract

We investigated whether chimpanzees (Pan troglodytes) misperceived food portion sizes depending upon the context in which they were presented, something that often affects how much humans serve themselves and subsequently consume. Chimpanzees judged same-sized and smaller food portions to be larger in amount when presented on a small plate compared to an equal or larger food portion presented on a large plate and did so despite clearly being able to tell the difference in portions when plate size was identical. These results are consistent with data from the human literature in which people misperceive food portion sizes as a function of plate size. This misperception is attributed to the Delboeuf illusion which occurs when the size of a central item is misperceived on the basis of its surrounding context. These results demonstrate a cross-species shared visual misperception of portion size that affects choice behavior, here in a nonhuman species for which there is little experience with tests that involve choosing between food amounts on dinnerware. The biases resulting in this form of misperception of food portions appear to have a deep-rooted evolutionary history which we share with, at minimum, our closest living nonhuman relative, the chimpanzee.

Figure 1

A. The Delboeuf illusion occurs when two same-sized central circles are perceived to be different depending upon the context in which they are presented. The two equal-sized black circles are perceived differently so that when a larger concentric circle encompasses the test circle (left), the test circle is thought to be smaller than when encircled by a relatively smaller concentric circle (right). B. Test stimuli for the Standard Illusion trials are shown where equal-sized continuous food portions are presented on a large and small plate so that the food on the large plate (left) appears to be smaller than the food on the small plate (right). C. Test stimuli for the Non-Standard Illusion trials are shown where a larger discrete food portion on a large plate (10 g of cereal - left) appears to be equal to the smaller portion on the small plate (8 g of cereal - right).

Figure 2

A. The percentage of small plate choices in the Standard Illusion trials. All chimpanzees had a significant preference (from chance, depicted by the dashed line) for the small plate over the large plate when both plates contained an equal portion of food. B. The percentage of large food portion choices in control trials (gray bars) versus Non-Standard Illusion trials (black bars). All chimpanzees selected the larger food portion less often in the Illusion trials when presented on different-sized plates than in control trials when presented on same-sized plates, even though both conditions presented the same relative difference in food portions.