Emily H. Harris

Perception of Food Amounts by Chimpanzees Based on the Number, Size, Contour Length and Visibility of Items.

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.

Quantity judgments of sequentially presented food items by capuchin monkeys (Cebus apella)

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.

Discrimination reversal learning in capuchin monkeys (Cebus apella).

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 for performance, 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.

Ordinal judgments of symbolic stimuli by capuchin monkeys (Cebus apella) and rhesus monkeys (Macaca mulatta): The effects of differential and nondifferential reward.

Ordinal learning was investigated in capuchin monkeys (Cebus apella) and rhesus monkeys (Macaca mulatta). In Experiment 1, both species were presented with pairings of the Arabic numerals 0 to 9. Some monkeys were given food rewards equal to the value of the numeral selected and some were rewarded with a single pellet only for choosing the higher numeral within the pair. Both species learned to select the larger numeral, but only rhesus monkeys that were differentially rewarded performed above chance levels when presented with novel probe pairings. In Experiment 2, the monkeys were first presented with arrays of 5 familiar numerals (from the range 0 to 9) and then arrays of 5 novel letters (from the range A to J) with the same reward outcomes in place as in Experiment 1. Both species performed better with the numerals, suggesting that an ordinal sequence of all stimuli had been learned during Experiment 1, rather than a matrix of two-choice discriminations.

Ordinal judgments and summation of nonvisible sets of food items by two chimpanzees and a rhesus macaque.

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.

Macaques’ (Macaca mulatta) use of numerical cues in maze trials

We tested the ability of number-trained rhesus monkeys to use Arabic numeral cues to discriminate between different series of maze trials and anticipate the final trial in each series. The monkeys’ prior experience with numerals also allowed us to investigate spontaneous transfer between series. A total of four monkeys were tested in two experiments. In both experiments, the monkeys were trained on a computerized task consisting of three reinforced maze trials followed by one nonreinforced trial. The goal of the maze was an Arabic numeral 3, which corresponded to the number of reinforced maze trials in the series. In experiment 1 (n=2), the monkeys were given probe trials of the numerals 2 and 4 and in experiment 2 (n=2), they were given probe trials of the numerals 2–8. The monkeys receiving the probe trials 2 and 4 showed some generalization to the new numerals and developed a pattern of performing more slowly on the nonreinforced trial than the reinforced trial before it for most series, indicating the use of the changing numeral cues to anticipate the nonreinforced trial. The monkeys receiving probe trials of the numerals 2–8 did not predict precisely when the nonreinforced trial would occur in each series, but they did incorporate the changing numerals into their strategy for performing the task. This study provides the first evidence that number-trained monkeys can use Arabic numerals to perform a task involving sequential presentations.

Ordinal-list integration for symbolic, arbitrary, and analog stimuli by rhesus macaques (Macaca mulatta).

Two numeral-trained monkeys learned to produce 3 5-item lists of Arabic numerals, colors, and arbitrary signs in the correct sequence. The monkeys then responded at above-chance levels when the authors tested them with nonrewarded pair-wise comparisons of items from different lists, indicating their use of ordinal-position information. The authors also tested the monkeys with nonrewarded pair-wise comparisons of an analog quantity and an item from 1 of the 3 learned lists. Although the monkeys were not trained to serially order analog quantities, 1 monkey correctly integrated the analog quantities with the lists of numerals, colors, and signs. The consistent use of an ordinal rule, despite different types of training and varying degrees of experience with the 4 types of stimuli, suggested that the monkey had a robust concept of ordinality.

What Do Arabic Numerals Mean to Macaques (Macaca mulatta)

In the past, rhesus macaques (Macaca mulatta) have demonstrated an ability to use Arabic numerals to facilitate performance in a variety of tasks. However, it remained unclear whether they understood the absolute and relative values of numerals. In Experiment 1, numeral-trained macaques picked the larger stimuli when presented with pairwise comparisons involving numerals and analog quantities. In Experiment 2, macaques were provided with numeral cues indicating the number of times a behavior could be performed in one location for a reward. Of the 4 monkeys, 3 performed above chance, but they often erred by performing more behaviors than indicated. The results of these studies indicate that the monkeys have knowledge of the approximate quantities represented by each numeral.

Rhesus monkeys (Macaca mulatta) select Arabic numerals or visual quantities corresponding to a number of sequentially completed maze trials

Four number-trained rhesus monkeys were trained to enumerate their sequential responses. After completing a series of computerized maze trials, the monkeys were given a same/different discrimination involving a numerical stimulus (an Arabic numeral or a visual quantity) and the letterD. The goal was to choose the numerical stimulus if it matched the number of just-completed maze trials, and to choose the letterD if it did not. There were large individual differences in performance, but one animal performed above 70% when receiving randomly intermixed series of 1, 3, 5, and 9 maze trials. This indicates that the monkey was keeping track of the approximate number of maze trials completed in each series and using that numerical cue to respond during the same/different discrimination.