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

Summary

A tale of two species View from above of the apparatuses used for ring doves (A) and guppies (B). Credit: M. Santaca et al., 2025 The authors tested 38 ring doves and 19 guppies (of the “snakeskin cobra green” ornamental strain) for their experiments. The doves were placed in a testing cage with the bottom covered with an anti-slip wooden panel and a branch serving as a perch and starting point; the feeding station was at the opposite end of the cage. The guppies were tested in single tanks with a gravel bottom. In both cases, the test subjects were presented with visual stimuli in the form of two white plastic cards. Sizes differed for the doves and the guppies, but each card showed an array of six black circles with a bit of food serving as the center “circle”: red millet seeds for the doves and commercial flake food for the guppies. The circles were smaller on one of the cards and larger on the other. The subjects were free to choose food from one of the cards, and the card with the unchosen food was removed promptly. If no choice was made after 15 minutes, the trial was null and the team tried again after a 15-minute interval. The authors found that the guppies were indeed highly susceptible to the Ebbinghaus illusion, choosing food surrounded by smaller circles much more frequently, suggesting they perceived it as larger and hence more desirable. The results for ring doves were more mixed, however: some of the doves seemed to be susceptible while others were not, suggesting that their perceptual strategies are more local, detail-oriented, and less influenced by their surrounding context. “The doves’ mixed responses suggest that individual experience or innate bias can strongly shape how an animal interprets illusions,” the authors concluded. “Just like in humans, where some people are strongly fooled by illusions and others hardly at all, animal perception is not uniform.” The authors acknowledge that their study has limitations. For instance, guppies and ring ...