Repository 3-2001 Size Matters : Impact of Item Size and Quantity on Array Choice by Chimpanzees ( Pan troglodytes )

The authors previously reported that chimpanzees (Pan troglodytes) showed a striking bias to select the larger of 2 candy arrays, despite a reversed reward contingency in which the animals received the smaller, nonselected array as a reward, except when Arabic numerals were used as stimuli. A perceptual or incentive-based interference occurred that was overcome by symbolic stimuli. The authors of the present study examined the impact of element size in choice arrays, using 1 to 5 large and small candies. Five test-sophisticated chimpanzees selected an array from the 2 presented during each trial. Their responses were not optimal, as animals generally selected arrays with larger total mass; thus, they received the smaller remaining array as a reward. When choice stimuli differed in size and quantity, element size was more heavily weighted, although choices reflected total candy mass. These results replicate previous findings showing chimpanzees' difficulties with quantity judgments under reverse reward contingencies and also show that individual item size exerts a more powerful interference effect. Previous studies of chimpanzees' ability to make relative size judgments of food items were first reported by Menzel (1960, 1961) and colleagues (Menzel & Davenport, 1962; Menzel & Draper, 1965). Results indicated that the chimpanzees could reliably select the largest portion from among the available choices without specific training. We have also found that relative quantities of stimuli are important determinants of choices by chimpanzees such that the disposition to select a larger quantity can interfere with the chimpanzees' selection of the smaller array when a reverse reward contingency is enforced. Under these conditions, to maximize reward, the chimpanzees should choose the smaller array, which would be discarded, and the chimpanzees would be rewarded with the remaining, larger array. This interference effect has been highly persistent over hundreds of trials, over several years, and through numerous versions of the task (see Boysen & Berntson, 1995; Boysen, Berntson, Hannan, & Cacioppo, 1996). If, however, arrays were replaced by number symbols (all chimpanzees had expertise with the functional use of Arabic numerals), the chimpanzees were able to reliably select the smaller numeral, and under the imposed reverse contingency, received the larger reward. This more optimal performance was shown immediately once numbers were used, although in subsequent sessions when candy arrays were reintroduced, all chimpanzees' performances reverted to the original strategy of choosing the larger array (Boysen et al., 1996). Results suggested that a powerful, perceptually driven predisposition was competing with the acquired associative effects of the reinforcement contingencies, and this interference effect has been replicated with three other species, including Japanese macaque monkeys (Silberburg & Fujita, 1996), pigeons (Emmerton, 1995), and more recently, squirrel monkeys (Anderson, Awazu, & Fujita, 2000). The ability of older children and some nonhuman subjects to take into account the divergent perceptual features of a conservation task shares some of the same requirements of our quantity judgment task. However, in the quantity judgment task, there was no physical transformation of the stimuli during testing. Instead, the perceptual discrepancies were inherent in the physical and incentive properties of the comparison arrays, including quantity, valence, representational modality, or some combination of the three (Boysen & Berntson, 1995; Boysen et al., 1996; Boysen, Berntson, & Mukobi, 1999). Of further interest are other potential perceptual features such as size, which may also contribute to judgments between stimuli with inherent physical differences. In the present experiment, we examined the contribution of size to comparisons of food arrays, under the same reversed-contingency conditions as our original quantity judgment task.