Exploring the Solitaire Illusion in Guppies (Poecilia reticulata)

The solitaire illusion is a numerosity illusion that occurs when the spatial arrangement of items influences quantity estimation. To date, this illusion has been reported in monkeys, although it seems to be weaker compared with its prevalence in humans, and no study has investigated whether nonprimate species perceive it. In the present work, we asked whether a more distantly related species, fish, perceived the solitaire illusion. To achieve this goal, adult guppies (Poecilia reticulata) were trained to select the array containing the larger quantity of black dots in the presence of 2 mixed arrays containing white and black dots. After reaching the learning criterion, guppies were presented with novel dot quantities, including test trials with 2 solitaire arrangements. The overall performance of the subjects indicated that they perceived the illusion, although analyses at the individual level indicated interindividual differences. These results align with recent evidence from nonhuman primates suggesting that distantly related species also may perceive this illusion, even though numerosity misperception arising from the solitaire arrangement appears to be less robust than in human and nonhuman primates.