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Can angelfish (Pterophyllum scalare) count? Discrimination between different shoal sizes follows Weber’s law

Abstract

The ability to discriminate between larger and smaller quantities has been demonstrated in several mammalian and avian species suggesting the possibility of evolutionary conservation of this characteristic. Preference for the larger of two groups has also been shown in fish species, although this ability has rarely been systematically studied in lower order vertebrates, and thus the mechanisms of such ability are not understood. Here, we exploit the tendency of angelfish to seek protection in an unfamiliar environment by joining a group of conspecifics, a behaviour called shoaling. Test fish were given a simultaneous choice between shoals varying both in terms of numerical ratios and absolute numbers of fish. Our results provide evidence for quantity discrimination in angelfish. In general, experimental subjects chose the larger of two shoals. Furthermore, in agreement with Weber’s law, which holds that discrimination between two quantities depends on their ratio, the discrimination between shoals of different quantities of fish was more difficult when the shoal sizes became more similar. The limit of discrimination ratio was found to be below 2:1. Briefly, angelfish are able to discriminate between different quantities of conspecifics subject to a ratio limit, a finding that implies a fitness component in this behaviour similar to what has been demonstrated in higher order vertebrates.

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Acknowledgments

We are grateful to three anonymous reviewers for their useful comments on the manuscript. R. G. was supported by a NSERC (Canada) grant. The University of Oviedo provided a visiting grant to L.M.G-L. The experiments described here comply with the current laws of the country (Spain) in which they were performed. The authors declare that they have no conflict of interest.

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Correspondence to Luis M. Gómez-Laplaza or Robert Gerlai.

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Gómez-Laplaza, L.M., Gerlai, R. Can angelfish (Pterophyllum scalare) count? Discrimination between different shoal sizes follows Weber’s law. Anim Cogn 14, 1–9 (2011). https://doi.org/10.1007/s10071-010-0337-6

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  • DOI: https://doi.org/10.1007/s10071-010-0337-6

Keywords

  • Numerical cognition
  • Quantity discrimination
  • Fish
  • Shoaling
  • Group choice
  • Numerical ratios