Guppies fall for a classic optical illusion. Doves, usually, do too
Summary: Comparing animals’ susceptibility to optical illusions can show how perception evolved.
Classic Illusion: The Müller–Lyer Effect
The Müller–Lyer illusion, first described in the 19th century, tricks the eye into judging two identical lines as unequal when they are framed by arrowheads pointing inward or outward. In humans, the line with outward-pointing arrows appears longer, revealing how our brains use contextual cues to interpret depth and distance. The illusion has become a standard test for perceptual processing across species.
Guppies Show the Same Bias
In a recent study published in *Animal Cognition*, researchers trained guppies (Poecilia reticulata) to choose between two lines in a maze. Despite their small brains, the fish consistently favored the line with outward-pointing arrowheads, mirroring the human pattern. The experiment ruled out color or motion cues, confirming that guppies rely on similar spatial heuristics when making decisions.
Doves Confirm the Pattern Across Species
Parallel experiments with common doves (Columba livia) demonstrated the same perceptual bias. Birds pecked at the “longer” line more often, even when the task was framed as a food reward. This cross‑taxonomic consistency suggests that the underlying neural circuitry for interpreting line length may be deeply conserved, likely dating back to early vertebrate ancestors.
Implications for the Evolution of Vision
These findings imply that perceptual shortcuts—like the Müller–Lyer effect—are not unique to humans but are widespread across vertebrates. Such heuristics likely evolved to accelerate decision‑making in complex environments, trading precise measurement for speed. Future research will explore whether similar biases exist in mammals, reptiles, and cephalopods, shedding light on the evolutionary pressures that shaped visual processing.