Optical illusions in different ungulates species

Abstract

Optical illusions have been used in behavioural studies in recent years to investigate the perceptual mechanisms underlying vision in animals. Although susceptibility to some illusions has been extensively studied in some species, only two studies have yet been carried out in ungulates, suggesting that they may perceive optical illusions in a way similar to humans. Here, we used two food-choice tasks to study susceptibility to the Müller-Lyer and Delboeuf illusions in 17 captive individuals belonging to four ungulate species (Lama guanicoe, Lama glama, Ovis aries, Capra hircus). In control trials, individuals reliably preferred the longer/larger food over the shorter/smaller one. In experimental trials, individuals significantly preferred the food stick between two inward arrowheads over an identical one between two outward arrowheads in the Müller-Lyer task, and they preferred the food on the smaller circle over an identical one on the larger circle in the Delboeuf task, with no significant differences across species. Overall, these findings suggest that, in line with our predictions, ungulates are susceptible to the Müller-Lyer and the Delboeuf illusions in a similar way to humans, and indicate that the perceptual mechanisms underlying size estimation in arctiodactyla might be similar to those of other ungulate and primate species, including humans.