Quantification abilities in angelfish (Pterophyllum scalare): the influence of continuous variables
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Previous studies investigating quantity discrimination have shown that angelfish are able to select the larger of two groups of conspecifics (shoals). The discrimination limits shown by angelfish were similar to those found for other vertebrates when large (≥4) and small quantities (<4) were presented. However, in these studies, no attempt was made to control for non-numerical features of the stimulus shoals and thus the question whether numerical or some quantitative attributes of the shoals were utilized for making the choices could not be answered. Here, we investigate whether angelfish can discriminate between shoals differing in numerical size using non-numerical attributes. We systematically manipulate density, inter-fish distance, and overall space occupied by the shoals, one factor at a time, and analyse the choices angelfish made between the contrasting stimulus shoals. The stimulus shoals consisted of contrasts between large (10 vs. 5) and small (3 vs. 2) number of conspecifics. We found density to be a sufficient condition for discrimination between large shoals as the test subjects preferred the more dense shoal. Manipulation of inter-fish distance indicated that this variable is not a necessary factor in discrimination at either shoal size contrast. Likewise, we found that the size of space occupied by the contrasted shoals also did not significantly influence discrimination. Sensitivity to the density of large shoals indicates that angelfish can discriminate shoal size using this non-numerical cue. Nevertheless, the factors we examined may represent only a subset of all possible non-numerical features upon which angelfish may base their discrimination. Thus, we suggest that further research is required to clarify whether and under what circumstances angelfish may use numerical or non-numerical features when discriminating between shoals of differing size.
KeywordsQuantity discrimination Continuous variables Angelfish Shoal choice Numerical cognition
We would like to thank anonymous reviewers for their valuable comments and insightful suggestions for improving the manuscript. We thank Miguel Sánchez Santillán for his help during experiments. This research was supported by grant MICINN-10-PSI2010-16487 from the Ministerio de Ciencia e Innovación (Spain) to L.M.G.-L., and an NSERC (Canada) Discovery grant to R.G. The experiments described here comply with the current laws of the country (Spain) in which they were performed.
Conflict of interest
The authors declare that they have no conflict of interest.
- Binoy VV, Thomas KJ (2004) The climbing perch (Anabas testudineus Bloch), a freshwater fish, prefers larger unfamiliar shoals to smaller familiar shoals. Curr Sci 86:207–211Google Scholar