Figure 2. The experimental apparatus used in the fish experiment. Fish were individually placed into the middle of the apparatus where two shoals containing different numbers of fish were visible at the ends. doi:10.1371/journal.pone.0031923.g002 the juvenile fish was similar to that used for the adult fish but reduced in size. The central ‘subject tank’ was 20617.5625 cm and the two ‘stimulus tanks’ were semi-octagonal shaped, with a side of 6.3 cm. This apparatus was filled with 4 cm of water and was partially modified when testing the 40-day-old fish by enlarging the stimulus tanks (semi-octagonal sides 8.3 cm long) and increasing the level of water (6 cm). Similar Numerical Systems in Humans and Guppies Two shoals containing different numbers of fish were placed into the stimulus tanks. The subjects were individually introduced into a transparent plastic cylinder (10 cm in diameter for the adults, and 3.5 cm for the juveniles) in the middle of the subject tank and allowed to acclimatize for two minutes. After this period the subject was observed for 15 minutes. Shoal preference was calculated as the time spent by the subject within a distance of 11 cm (4 cm when testing juveniles) from the glass facing either of the stimulus tanks. Subjects that did not visit either stimulus sector at least three times or spent less than 50% of the time in a choice area were considered inactive; they were discarded and replaced by another fish. The same five numerical ratios of the students’ experiment were presented to fish both for small (1 vs. 4, 1 vs. 3, 1 vs. 2, 2 vs. 3 and 3 vs. 4) and large (4 vs. 16, 4 vs. 12, 4 vs. 8, 4 vs. 6 and 6 vs. 8) numerical contrasts. At the end of these trials, we increased the sample size for ratio 0.67 by testing 64 additional adult females, 32 in a small quantity discrimination (2 vs. 3) and 32 in a large quantity discrimination (4 vs. 6). Results Undergraduate experiment Small numbers. In accordance with the theory, reaction time was not affected by numerical ratio (ANOVA F(4,68) = 1.474, P = 0.220, Fig. 3). Control of continuous variables did not affect Figure 3. The results of the undergraduate experiment. Accuracy (proportion of correct responses) and reaction time are plotted against the numerical ratio of the contrasts for both large and small number (1–4) ranges. The performance of the participants showed ratio sensitivity for large numbers and ratio insensitivity in the small number range. Bars represent the standard error. doi:10.1371/journal.pone.0031923.g003 PLoS ONE | www.plosone.org 4 February 2012 | Volume 7 | Issue 2 | e31923