Summation and quantity judgments of sequentially presented sets by capuchin monkeys (Cebus apella).

Capuchin monkeys (Cebus apella) were presented with two sets of food items, identical in food type but differing in number. Animals selected one set and were permitted to consume their choice. Set sizes ranged from 1 to 6 items. In experiment 1, each set was uncovered and recovered before a response was made, and the monkeys selected the larger set at high levels. Experiment 2 presented sets that had both visible and nonvisible food items in them at the time of the response, thus requiring the monkeys to sum the total amount of food that was available. The monkeys again selected the larger set with no decrement in performance. Overall, the data indicate that capuchins, like other more extensively studied primate species in this area of research, are responsive to quantitative differences between sets. Capuchins succeed in making these quantity judgments when sets are nonvisible at choice time and when summation of items must be performed, thus demonstrating coordination of quantification skills and memory. Capuchins also inhibit responses to visible food items when those items are only part of an overall smaller quantity of food compared with a completely nonvisible set.