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Food density and preferred quantity: discrimination of small and large numbers in angelfish (Pterophyllum scalare)


Many animal species share the ability to discriminate between sets with different quantity of food items. In fish, this ability has rarely been investigated, although findings have been obtained do indicate a preference, as in other animals, for sets with large over small quantities. The role played by food item size has also been found to be important in the discrimination. However, another potentially important non-numerical variable, food density, has not been investigated. In this study, we examined the influence of density (inter-item distance) in the decision-making process of food discrimination in angelfish (Pterophyllum scalare). In a binary choice task, we kept the number and size of food items constant, but contrasted a set containing food items spaced further apart (sparse set) to another set with food items spaced more closely (dense set). We conducted this analysis with sets in the small (3 vs 3 food items) and in the large number range (5 vs 5 food items) and also varied the specific spatial arrangements of the food items in the sets. Contrary to expectations, angelfish showed a preference for the sparse sets over the dense sets in the five vs five contrasts irrespective of the specific spatial arrangement, but exhibited no preference in case of the three vs three contrasts. Subsequently, we slightly lengthened the inter-item distance in the dense sets, and found preference for the dense over the sparse sets. Last, we further examined the potential effect of spatial configuration of the items in the sets, but found no effect of this latter factor. Overall, these results indicate that higher density of the contrasted food item sets significantly influences choice in angelfish, which prefer denser sets if a clear discriminability of each individual item within the sets is provided.

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Fig. 1

(Modified from Gómez-Laplaza et al. 2019)

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We are grateful to Laura Romero for providing the basis for Fig. 1, and Álvaro L. Caicoya for his invaluable help with the JASP computer software and obtaining the results of the BF. This research was supported by Grant MINECO-17-PSI2016-78249-P from the Ministerio de Economía, Industria y Competitividad (Spain) to LMG-L and NSERC (Canada) Discovery Grant (311637) to RG.

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Correspondence to Luis M. Gómez-Laplaza.

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All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. All procedures performed in studies involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted (Ethics Committee of the University of Oviedo, permit: 13-INV-2010.

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Gómez-Laplaza, L.M., Gerlai, R. Food density and preferred quantity: discrimination of small and large numbers in angelfish (Pterophyllum scalare). Anim Cogn (2020). https://doi.org/10.1007/s10071-020-01355-6

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  • Quantity discrimination
  • Density assessment
  • Food
  • Fish cognition