Food Quantity Discrimination in Angelfish (Pterophyllum scalare): The Role of Number, Density, Size and Area Occupied by the Food Items

Abstract

Quantity discrimination, the ability to identify, process, and respond to differences in number, has been shown in a variety of animal species and may have fitness value. In fish, the ability to distinguish between numerically different shoals has been well studied. However, little work has been devoted to the investigation of such ability in a foraging context. Nevertheless, angelfish (Pterophyllum scalare) have been previously shown to be able to discriminate numerically different sets of food items, with variables such as size and density of the food items playing important roles in making the choice. Here, we examine the possible role of other numerical and non-numerical variables. Using a spontaneous binary choice task, we contrasted sets of food items differing in specifically controlled ways: (1) different numerical size but equal inter-item distance; (2) different numerical size and different inter-item distance; and (3) identical total contour length and area occupied but different individual food size and inter-food distance between the contrasted food sets. In Experiment 1, angelfish were found to prefer the sets with a large number of food items. In Experiment 2, they preferred the numerically smaller sets with clustered items to the numerically larger sets with scattered items, but only when the sets were in the large number range (10 vs. 5 food items). Finally, in Experiment 3 fish preferred numerically smaller sets with large-sized and scattered food items in the large number range sets. We conclude that food item number, density, and size may not be considered individually by angelfish, but instead, the fish respond to all these factors attempting to maximize energy gained from eating the food while minimizing energy expenditure collecting and/or protecting the food.

Keywords: angelfish; continuous variables; fish cognition; foraging; quantity discrimination.

Figure 1

Schematic top view representation of the experimental aquarium. The home and test compartments were separated by a smaller middle compartment (10 × 30 × 33 cm, length × width × depth) whose central part constituted the start box (10 × 10 × 33 cm length × width × depth). Partitions, with guillotine windows, delimited the compartments; panels (placed outside the experimental aquarium) where the stimuli were presented; and a transparent plastic divider that delimited the two preference zones (10.5 × 15 cm length × width) are indicated. Panels, also with guillotine windows (dashed line), were superimposed on the partitions when tests began, the experimenter could raise or lower these panels to open or close the guillotine windows. The guillotine window of the transparent partition delimiting the testing compartment was 14.5 cm away from but in front of, the transparent divider. The position of the home compartment and the testing compartment was counterbalanced to prevent any lateral side bias.

Figure 2

Examples of the contrasting food sets tested in each of the experiments and the corresponding numerical comparisons. Drawings are not at scale.

Figure 3

Choice of angelfish in the different numerical contrasts tested when the size and density (inter-item distance) of the items in each contrast were equal. The proportion of time (preference index) spent by test fish in the preference zone close to the food sets. Box plots show median (horizontal line in the boxes), 25% and 75% quartiles (boxes), and the highest and lowest values within the range of 1.5 times the respective quartiles (whiskers). Blades represent the mean proportion value. Values above 0.5 indicate a preference for the numerically larger food sets. A significant departure from the null hypothesis of no preference is indicated by asterisks, *p < 0.05.

Figure 4

Choice of angelfish in the different numerical contrasts when the size of the items was equal but the density (inter-item distance) was different: clustered (C) and scattered (S). The proportion of time (preference index) spent by test fish in the preference zone close to the food sets. Box plots show median (horizontal line in the boxes), 25% and 75% quartiles (boxes), and the highest and lowest values within the range of 1.5 times the respective quartiles (whiskers). Blades represent the mean proportion value. Values above 0.5 indicate a preference for the numerically larger, scattered food sets, whereas values below 0.5 indicate a preference for the numerically smaller, clustered food sets. A significant departure from the null hypothesis of no preference is indicated by asterisks, *p < 0.05.

Figure 5

Choice of angelfish in the different numerical contrasts when the convex hull and contour length was equated between the contrasting sets, but the size and density of the items were different. The proportion of time (preference index) spent by test fish in the preference zone close to the food sets. Box plots show median (horizontal line in the boxes), 25% and 75% quartiles (boxes), and the highest and lowest values within the range of 1.5 times the respective quartiles (whiskers). Blades represent the mean proportion value. Values above 0.5 indicate a preference for the numerically larger sets with clustered and small items, whereas values below 0.5 indicate a preference for the numerically smaller sets, with scattered and large items. A significant departure from the null hypothesis of no preference is indicated by asterisks, *p < 0.05. SL, scattered and large food items; CS, clustered and small food items.