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 judgements 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 influence 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 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 judgements 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.