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

Abstract

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.

Figure 1

Overall performance in choosing the larger amount in Experiment 1. Data are shown on the basis of the numerical ratio between sets and the ratio between the set with the smaller total amount of food compared to the set with the larger total amount of food. Error bars represent standard errors of means.

Figure 2

Overall performance in choosing the larger amount for each of the trial types in Experiment 1. Data are separated on the basis of assuming the number, contour, or the location of the biggest individual piece of food acted as the primary cue. Data also are separated on the basis of how well those cues would have operated in helping the chimpanzee choose the larger amount of food. Equal trials (Eq.) were those in which the cue was equated in both set. Congruent trials (Con.) were those in which the cue indicated the set with the larger total amount. Incongruent (Inc.) trials were those in which the cue indicated the set with the smaller total amount of food. Error bars represent standard errors of means.

Figure 3

Overall performance in choosing the larger amount for each condition in Experiment 2 for which one set contained more food than the other. See text for descriptions of each trial type. Error bars represent standard errors of means.

Figure 4

Overall performance in choosing the larger amount in Experiment 3. Data are shown on the basis of the numerical ratio between sets and the ratio between the set with the smaller total amount of food compared to the set with the larger total amount of food. Error bars represent standard errors of means.

Figure 5

Overall performance in choosing the larger amount for each of the trial types in Experiment 3. Data are shown as in Figure 2. Error bars represent standard errors of means.