Protective polymorphism in populations of computer-simulated moth-like prey

Colour pattern polymorphism may provide protection to polymorphic prey by reducing the rate of predation by visual predators. We tested whether this effect was dependent on the level of crypsis of the prey and the density of the population by presenting 40 human subjects with computer generated prey on a touch-sensitive screen. A complex heterogeneous background was generated composed of randomly distributed coloured pixels. Prey items, designed to resemble resting moth-like Lepidoptera, and composed of similarly coloured pixels, were presented simultaneously against the background in populations that were either monomorphic (every prey item had the same colour pattern) or highly polymorphic (each prey item had a unique colour pattern). Crypsis was controlled by altering the number of colours in the prey; density was controlled by altering the number of prey presented in a population. We found a significant effect of polymorphism on time to delete prey when subjects were unfamiliar with the prey items but no interactions between level of polymorphism and either crypsis or density. These results argue against the hypothesis that polymorphism affords greatest protection to cryptic polymorphic prey: polymorphism may be protective irrespective of level of crypsis.