Stimulus salience as an explanation for imperfect mimicry.

The theory of mimicry explains how a mimic species gains advantage by resembling a model species [1-3]. Selection for increased mimic-model similarity should then result in accurate mimicry, yet there are many surprising examples of poor mimicry in the natural world [4-8]. The existence of imperfect mimics remains a major unsolved conundrum. We propose and experimentally test a novel explanation of the phenomenon. We argue that predators perceive prey as having several traits, but that the traits differ in their importance for learning. When predators learn to discriminate prey, high-salience traits overshadow other traits, leaving them under little or no selection for similarity, and allow imperfect mimicry to succeed. We tested this idea experimentally, using blue tits as predators and artificial prey with three prominent traits: color, pattern, and shape. We found that otherwise imperfect color mimics were avoided about as much as perfect mimics, whereas pattern and shape mimics did not gain from their similarity to the model. All traits could separately be perceived and learned by the predators, but the color trait was learned at a higher rate, implying that it had higher salience. We conclude that difference in salience between components of prey appearance is of major importance in explaining imperfect mimicry.