Similar Numerical Systems in Humans and Guppies Figure 1. The experimental design used in the undergraduate experiment. The participants were sequentially presented with two groups of dots and had to estimate which group was more numerous. doi:10.1371/journal.pone.0031923.g001 perform easy discriminations [17,40,41]. In these cases, in fact, participants commit few errors and variation in accuracy is potentially obscured by a ceiling effect. However since accuracy (proportion of time spent near the larger shoal) was the only variable measured in the fish experiment, for comparison, we also analyzed accuracy in the human experiment. Fish experiment Like many other social fish, single guppies placed in an unknown environment show a strong tendency to join social companions and, if choosing between two shoals, they exhibit a preference for the larger one [37,42]. This spontaneous tendency was used in this experiment to measure the ability of guppies to discriminate between two numerosities. During their life guppies might have different opportunity of familiarizing with large or small shoals and this could potentially affect the experiment. Because of this, alongside a sample of 140 adults, we tested a sample of 200 immature fish reared in pairs and therefore with no previous experience of social groups. Subjects. The experienced subjects were adult females because they are more gregarious than males. They were reared in groups of 15 or more individuals. Seventy fish (14 in each numerical contrast) were tested in small quantity discriminations, and 70 were tested in large quantity discriminations. The inexperienced subjects were juvenile fish tested at the onset of their numerical abilities. A recent study showed that one-day-old guppies could discriminate the larger shoal when the choice was between numbers in the small number range, whereas the ability to discriminate large quantities appeared later, at approximately day 40 [37]. Thus, 100 one-day-old fish (20 in each numerical contrast) were tested in small quantity discrimination tasks, and 100 40-day-old fish were tested in large quantity discrimination tasks. Stimuli and procedure. The experimental apparatus was composed of three adjacent tanks (Fig. 2). The central one, the ‘subject tank’, housed the test fish (36660635 cm). At the two ends two ‘stimulus tanks’ (36610610 cm) faced the subject tank. The apparatus was filled with 10 cm of water. The apparatus for PLoS ONE | www.plosone.org 3 February 2012 | Volume 7 | Issue 2 | e31923