performances (F(1,17) = 0.421, P = 0.525; interaction F(4,68) = 0.822, P = 0.516). To test if there was a significant slope, we performed a polynomial trend analysis [43]. No significant linear or larger order trend was found (F(1,17) = 2.644, P = 0.122). A likelihood ratio analysis (see [44] for details) also reflected the absence of ratio effect (l = 3.67). Comparable differences were observed in accuracy. Accuracy was not affected by numerical ratio (F(4,68) = 0.572, P = 0.684), continuous variables did not affect the performance (F(1,17) = 1.285, P = 0.273) and the interaction was not significant (F(4,68) = 0.383, P = 0.820). No significant trend was found (F(1,17) = 0.378, P = 0.547). A likelihood ratio analysis also reflected the absence of ratio effect (l = 1.15). Large numbers. In the large quantity range the reaction time increased with decreasing numerical ratio (F(4,68) = 31.889, P,0.001), while the continuous variable factor was not significant (F(1,17) = 2.880, P = 0.108; interaction F(4,68) = 2.469, P = 0.053, Fig. 3). There was a significant trend (linear trend: F(1,17) = 57.302, P,0.001; quadratic trend: F(1,17) = 8.250, P = 0.011). Similarly, accuracy decreased with decreasing numerical ratio (F(4,68) = 8.564, P,0.001), control of the continuous variables did not affect performance (F(1,17) = 0.205, P = 0.657) and no interac- tion was found (F(4,68) = 0.359, P = 0.837). There was a significant trend (linear trend: F(1,17) = 24.348, P,0.001; quadratic trend: F(1,17) = 8.303, P = 0.010). Similar Numerical Systems in Humans and Guppies On the whole, accuracy did not differ between the small number range and the large number range (t(17) = 1.877, P = 0.078). However, for the ratio of 0.75 the participants were significantly more accurate in the small number range than the large number range (t(17) = 2.197, P = 0.042). Fish experiment Data were analysed separately for small number and large number range with a two (experience: juveniles/adults) by five (numerical ratio: 0.25/0.33/0.50/0.67/0.75) between-subject ANOVA. Small numbers. The proportion of time spent near the larger shoal was not influenced by either numerical ratio (F(4,169) = 0.047, P = 0.996) or experience (F(1,169) = 0.030, P = 0.864), and the interaction was not significant (F(4,169) = 0.242, P = 0.914, Fig. 4). No significant trend was found (F(4, 169) = 0.045, P = 0.876). Likelihood ratio analysis also reflected the absence of ratio effect (l = 3.68). Large numbers. In the large number range, the proportion of time spent near the larger shoal was influenced by numerical ratio (F(4,169) = 3.190, P = 0.015) but not by experience (F(1,169) = 0.300, P = 0.585; interaction: F(4,169) = 0.341, P = 0.850, Fig. 4). There was a significant linear trend (F(4, 169) = 3.312, P,0.001). Figure 4. The results of the fish experiment. The proportion of time spent near the larger shoal is plotted against the numerical ratio of the contrasts for both large and small number (1–4) ranges. The performance of the fish (both experienced and inexperienced fish) adhered to the same patterns as for humans in the two numerical ranges, with ratio sensitivity only being shown in the large number range. Bars represent the standard error. doi:10.1371/journal.pone.0031923.g004 PLoS ONE | www.plosone.org 5 February 2012 | Volume 7 | Issue 2 | e31923