The more eye-like, the more intimidating-visual illusion gives new evidence for the anti-predation mechanism of eyespots.

Eyespots have been shown to perform an anti-predator function by intimidating predators. However, whether predators are deterred by the eyespots' mimicking eyes of higher-order predators, or by the conspicuousness of the spots, is still being debated. The anti-predator mechanism of eyespots, which involves the predator's visual perception and cognition, urges to be examined. To explore the "eye mimicry hypothesis" and "conspicuous signal hypothesis," we designed prey with deceptive eyespots eliciting 2-dimensional (2D) or 3-dimensional (3D) visual illusions for domestic chicks (as predators). For 2D visual illusion, we provided prey with occluded eyespots for chicks. Chicks can "complete" the occluded eyespots into a full 2D concentric circle by the amodal completion mechanism, that is, one of visual illusion. For 3D visual illusion, we created the crescent-shaped sparkles on the eyespots to simulate a visually 3D eye. Our result indicated that (1) 2D: chicks hesitated more when facing preys with occluded eyespots than facing preys with broken eyespots and (2) 3D: compared to prey with no-sparkle eyespots, chicks spent more time approaching prey with sparkle eyespots and were more likely to approach them tangentially. Thus, the visual illusion perceived by chicks would impact the anti-predation effectiveness of eyespots. Eyespots that more closely resemble the real eyes, especially those with 3-dimensionality, provided more effective protection. Our study supported the eye mimicry hypothesis from the perspective of visual illusion.