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.

Video playback versus live stimuli to assess quantity discrimination in angelfish (Pterophyllum scalare)

“…As in our previous studies (e.g., Gómez-Laplaza & Gerlai, 2020a , 2020b ), juveniles of the freshwater cichlid species, angelfish ( Pterophyllum scalare ), measuring about 3.3 cm standard length, were obtained from a local supplier. Only juveniles of this sexually monomorphic species were studied to avoid potential confounding effects arising from courtship interactions.…”

“…The computer, along with the corresponding monitor from where the presentation was viewed and controlled, was concealed behind a blind. A video length of 10 min was selected as angelfish have been found to express behavioral differences in choice tests within this time period (Gómez-Laplaza & Gerlai, 2020a , 2020b ). After the video was recorded, the fish that served as the stimulus were returned to their home tanks in the laboratory.…”

The Sense of Number in Fish, with Particular Reference to Its Neurobiological Bases

“…In spontaneous foraging tasks, angelfish showed to prefer the numerically larger food set as long as the items were sized identically, with an accuracy that depended on the numerical ratio between the two quantities [ 79 ]. However, variables such as the size and density of the food items played an important role [ 80 , 81 ], suggesting that numerical and continuous physical cues may not be considered separately but instead are combined by fish to maximize food intake [ 82 ].…”

Zebrafish Shoaling, Its Behavioral and Neurobiological Mechanisms, and Its Alteration by Embryonic Alcohol Exposure: A Review

“…However, we do know, for example, that zebrafish, similarly to other fish species, as well as other vertebrate species too, can distinguish shoals based upon the number of shoal members (Seguin and Gerlai, 2017 ). From this initial observation, numerous follow-up studies may be envisioned that could explore the behavioral mechanisms upon which zebrafish base their decision to choose or abandon particular shoals, similarly to how quantity discrimination abilities have been explored with another fish species, the freshwater angelfish (see the study by Gómez-Laplaza and Gerlai, 2020 in this special issue). Furthermore, although some pioneering studies have been conducted on the subject (e.g., Saverino and Gerlai, 2008 ), we know very little about how zebrafish determine who is a conspecific and who is a hetero-specific fish.…”