Nonhuman animals reliably select the largest of two or more sets of discrete items, particularly if those items are food items. However, many studies of these numerousness judgments fail to control for confounds between amount of food e.g., mass or volume) and number of food items. Stimulus dimensions other than number of items also may play a role in how animals perceive sets and make choices. Four chimpanzees (Pan troglodytes) completed a variety of tasks that involved comparisons of food items (graham crackers) that varied in terms of their number, size, and orientation. In Experiment 1, chimpanzees chose between two alternative sets of visible cracker pieces. In Experiment 2, the experimenters presented one set of crackers in a vertical orientation (stacked) and the other in a horizontal orientation. In Experiment 3, the experimenters presented all food items one-at-a-time by dropping them into opaque containers. Chimpanzees succeeded overall in choosing the largest amount of food. They did not rely on number or contour length as cues when making these judgments but instead primarily responded to the total amount of food in the sets. However, some errors reflected choices of the set with the smaller total amount of food but the individually largest single food item. Thus, responses were not optimal because of biases that were not related to the total amount of food in the sets.

Recent assessments have shown that capuchin monkeys, like chimpanzees and other Old World primate species, are sensitive to quantitative differences between sets of visible stimuli. In the present study, we examined capuchins' performance in a more sophisticated quantity judgment task that required the ability to form representations of food quantities while viewing the quantities only one piece at a time. In three experiments, we presented monkeys with the choice between two sets of discrete homogeneous food items and allowed the monkeys to consume the set of their choice. In Experiments 1 and 2, monkeys compared an entirely visible food set to a second set, presented item-by-item into an opaque container. All monkeys exhibited high accuracy in choosing the larger set, even when the entirely visible set was presented last, preventing the use of one-to-one item correspondence to compare quantities. In Experiment 3, monkeys compared two sets that were each presented item-by-item into opaque containers, but at different rates to control for temporal cues. Some monkeys performed well in this experiment, though others exhibited near-chance performance, suggesting that this species' ability to form representations of food quantities may be limited compared to previously tested species such as chimpanzees. Overall, these findings support the analog magnitude model of quantity representation as an explanation for capuchin monkeys' quantification of sequentially presented food items.

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.

Two chimpanzees and a rhesus macaque rapidly learned the ordinal relations between 5 colors of containers (plastic eggs) when all containers of a given color contained a specific number of identical food items. All 3 animals also performed at high levels when comparing sets of containers with sets of visible food items. This indicates that the animals learned the approximate quantity of food items in containers of a given color. However, all animals failed in a summation task, in which a single container was compared with a set of 2 containers of a lesser individual quantity but a greater combined quantity. This difficulty was not overcome by sequential presentation of containers into opaque receptacles, but performance improved if the quantitative difference between sizes was very large.

Learning styles in capuchin monkeys were assessed with a computerized reversal-learning task called the mediational paradigm. First, monkeys were trained to respond with 90% accuracy on a two-choice discrimination (A+B-). Then the authors examined differences in performance on three different types of reversal trials (A-B+, A-C+, B+C-), each of which offered differing predictions forperformance, depending on whether the monkeys were using associative cues or rule-based strategies. Performance indicated that the monkeys mainly learned to avoid the B stimulus during training, as the A-C+ condition produced the best performance levels. Therefore, negative stimuli showed greater control over responding after reversal and reflected a more associative rather than rule-based form of learning.