Which animals are tricked by optical illusions?
Bowerbirds create scenes that make them appear larger to potential partners. Fish and butterflies may show what looks like a large staring eye. intimidate predators Or deflect attacks. Male peacock spiders raise their legs as part of a courtship ritual to make them appear much larger than they actually are.
These are just some of the strategies that help these animals survive and reproduce. They raise a fascinating question: are animals fooled by optical illusions?
Optical illusions are an important scientific tool because they reveal these shortcuts that the brain uses to transform raw sensory information into perceptions of reality. When an unexpected event occurs, scientists gain a better understanding of the rules that govern perception. If nonhuman animals are the subjects of these illusions, scientists could begin to better understand how evolution developed similar rules to improve survival and facilitate reproduction.
“Many animals use visual strategies such as exaggerating size or camouflage, because perception is not about faithfully replicating reality, but about surviving.” Maria Santacaresearcher in animal behavior and cognition at the University of Vienna, told Live Science in an email.
Size illusions are perhaps the most famous visual tricks. Humans fall in love with them all the time. A classic of the genre is the Ebbinghaus illusion, which shows that a circle surrounded by smaller circles appears much larger than the same circle surrounded by larger circles.
Guppies also fall into the trap of this illusion. Santacà was the main author of a Study 2025which demonstrated that when a circle of food flakes was surrounded by smaller disks, fish chose them more often, as if there was actually more food in the circle. On the other hand, the ringed doves, tested with the same configuration using millet seeds, did not always fall into the trap of the illusion.
The likely explanation lies in the two species’ respective ecosystems, Santacà said. “Guppies live in dynamic underwater habitats with variable light and complex backgrounds, so their visual system emphasizes global processing, integrating the whole scene. Doves, in contrast, feed on small seeds on textured surfaces, requiring precise local discrimination. Their perception might therefore be optimized for detail rather than context, making them less prone to this particular illusion.”
Context matters
It turns out that being around an animal can amplify these illusions. Female fiddler crabs prefer males with large claws, but their attractiveness is relative. A male flanked by two rivals with smaller claws is more attractive to a female than the same male surrounded by larger neighbors. This context effect reflects the Ebbinghaus illusion and suggests that men can enhance their perceived attractiveness simply by courting less imposing neighbors.
“These strategies exploit how visual systems interpret context, helping animals appear larger to rivals and smaller to predators.” » argued Santancà. “In nature, what matters is not to be seen accurately, but to be perceived in the most advantageous way.”
Not all species follow the same scenario: Pigeons are subject to the Ebbinghaus effectbut in reverse, while baboons are completely unaffected by the illusion. Kelley argued that “this suggests that brains are wired differently across species, which is not surprising because of physiological variations and because the most relevant information may differ across species.”
Not only do animals perceive illusions, but some have become masters at creating these tricks. “Not only may men use their body features to appear tall (and therefore more attractive), but they may also use and/or modify their physical or social environment to change the perception of a woman’s size,” Kelley said.
Large male birds, for example, arrange small to large pebbles on the floor of their bower (an area they construct to impress females as part of a courtship ritual) to create an illusion of forced perspective, a 2010 study find. Objects that are further away should take up less space in the field of view than closer objects of the same size. From the female’s point of view, the fact that this is not true makes the arbor appear shorter and therefore the male larger.
Others are deceived by illusions about their own bodies. Octopuses can be fooled by a version of the “rubber hand illusion“, a trick long considered unique to humans. In experiments, researchers caressed a real octopus arm hidden from view and a fake octopus arm visible at the same time. When the fake arm was pinched, the octopus reacted as if its own arm had been attacked: it changed color or backed away. A similar experiment revealed that mice were also fooled by this illusion. The fact that the nervous systems of octopuses and rodents evolved completely separately from ours makes it all the more surprising that they are also subject to this illusion.
Camouflage as an illusion
Camouflage offers another example. Disruptive coloring uses high-contrast areas toward the edges of prey bodies to create false boundaries that confuse predators’ edge-detection systems. Countershading – which is common in fish, reptiles and mammals – classifies color from dark on top to light below. As the sun comes from above, light bellies are thought to make prey harder to spot from below. Likewise, a 2013 study found that the backs of dark prey are thought to blend in better with darker soil or ocean depths, confusing predators hunting from above.
“Countershading is probably so widespread because it solves a very fundamental problem: how to avoid detection by predators when directional light produces regions of brightness/darkness on the body,” Kelley said.
In the same way that the environment can distort size in the Ebbinghaus illusion, context also distorts brightness and color. A gray spot appears darker against a pale background — a phenomenon called simultaneous brightness contrast. Similar effects occur for color. Insects, fish And birds all exhibit these biases, suggesting a common mechanism for processing contrasting colors and shades. The illusion could be useful for courting men to help them look brighter or for animals that change color to stand out from the background.
Illusions demonstrate that perception is not a matter of perfect accuracy; it’s about what works in a given environment. As Kelley told Live Science: “At the end of the day, it’s always about survival and reproduction!”
For guppies, context integration can help evaluate rivals or partners in a vacillating flow. For doves, precision trumps context when pecking at seeds. When animals themselves deploy illusions, they exploit these neural shortcuts as survival strategies. The gap between reality and perception provides a rich space for evolution to do some of its most creative work.
