The Delboeuf illusion's bias in food choice of teleost fishes: an interspecific study

An optimal foraging strategy often requires identifying and choosing the larger amount of food in the presence of multiple options, in order to maximize food intake. Food quantity estimation frequently depends on the perceptual ability to segregate food from the surrounding background. In human and nonhuman animals, it has recently been shown that the perception of food size is affected by the size of the background on which food is presented, with a tendency to overestimate food items encircled by a small background. This perceptual bias resembles an illusory effect known as the Delboeuf illusion. We investigated whether this bias occurs in five fish species: zebrafish, Danio rerio, redtail splitfin, Xenotoca eiseni, angelfish, Pterophyllum scalare, Siamese fighting fish, Betta splendens, and three-spot gourami, Trichopodus trichopterus. In control trials, we observed their spontaneous preference for choosing the larger of two amounts of food. In test trials, two same-sized food amounts were presented on a small and a large plate, a pattern that causes the Delboeuf illusion in humans. Similarities in subjects’ choices between the five species were greater than differences, with a significant bias towards selecting the food amount on the large plate. This suggests the existence of a perceptual bias, although one operating in the opposite direction from that of humans, which might interfere with the accurate food size estimation necessary for an optimal foraging strategy.