Do animals fall for optical illusions? It’s complicated.

Many of us are familiar with the Ebbinghaus illusion, where a central circle's size appears altered based on the size of surrounding circles. This phenomenon raises an intriguing question: Are humans the only creatures affected by such optical illusions, or do animals experience them as well? A recent study published in the journal Frontiers in Psychology delves into this topic, suggesting that an animal's susceptibility to optical illusions may hinge on their sensory environment. Previous research has yielded mixed findings regarding which species can be fooled by these visual tricks. For instance, dolphins, chicks, and redtail splitfins have demonstrated a vulnerability to certain illusions, while others, like pigeons, baboons, and gray bamboo snakes, do not. One of the more amusing examples of animal behavior related to visual perception is the well-known tendency of cats to squeeze into boxes—an observation humorously termed the "if it fits, I sits" phenomenon. This behavior not only highlights a cat's preference for confined spaces but also provides insight into their visual processing. Research dating back to 1988, along with a more recent study in 2021, indicates that cats are indeed susceptible to the Kanizsa square illusion, suggesting they perceive subjective contours similarly to humans. The authors of the latest paper aimed to investigate the Ebbinghaus illusion in two distinct species: ring doves and guppies. Ring doves, which forage for small seeds on the ground, benefit from a keen attention to detail and precision in their visual judgments. Their binocular vision aids in accurately assessing size and distance. In contrast, guppies inhabit the dense and dynamic environments of tropical streams, facing challenges like fluctuating light and lurking predators. For guppies, rapid decision-making is crucial for survival, making an ability to quickly gauge relative size—akin to human global processing—a vital skill, especially in the context of the Ebbinghaus illusion.