Quantity discrimination by zebrafish (Danio rerio)

Evidence suggests that in humans numerousness judgements are based on two separate systems: one for representing small numbers (≤3-4, the object file system, OFS) and one for representing large numerical magnitudes (≥4, the analog magnitude system, AMS). Human infants seem to process small numbers via OFS (Feigenson and Carey 2005) and large numbers via AMS (Xu et al. 2005), with a gap in the processing of values at the boundary of the two domains. When 12 14-month-old infants were required to choose between two quantities of elements, they succeeded and chose the larger set with 1 versus 2, 1 versus 3, and 2 versus 3 but failed with 1 versus 4. Infants’ failure indicates that a different representation system operates over small numbers (Feigenson et al. 2002). Intriguingly, some recent data seem to suggest a similar phenomenon exists in fish (Piffer et al. 2012). However, contrasting data have been reported by Gómez- Laplaza and Gerlai (2011) on a different species of fish. Here, we developed a novel procedure to study numerousness discrimination in fish in an attempt to disentangle some of these contradictory results: Using the novel method, we first established a size-set limit for discrimination (using 1 vs. 2, 2 vs. 3, and 3 vs. 4 discriminations); We then tested fish for performance at discriminations between two numerousness spanning outside the boundary of the two presumed numerical systems, both with singular/plural (1 vs. 4) and plural (2 vs. 4) comparisons, as in experiments with infants.