Hierarchical overshadowing of stimuli and its role in mimicry evolution

An elegant study by Kazemi et al. (2014, Current Biology, 24, 965-969) found that blue tits, Cyanistes caeruleus, that had been trained to distinguish between rewarding and nonrewarding prey subsequently avoided novel prey that had the same colour (but not the same shape or pattern) as the nonrewarding prey. Their work suggested that certain high-salience discriminatory traits can overshadow other informative traits, allowing imperfect mimics to establish. Here we began by replicating their ambitious experiment by evaluating the behaviour of 320 human subjects foraging on computer-generated prey. However, to fully understand how overshadowing might facilitate the establishment of incipient mimics, we significantly extended their protocol to a full factorial design involving mimics with colour, pattern and/or shape in common with their former models. As Kazemi et al. reported, participants placed more weight on colour similarity than on shape similarity when rejecting prey. Two-trait mimics with the same colour and pattern or the same colour and shape as their former models were avoided as frequently as perfect mimics, while colour-only mimics were avoided more frequently than shape-only mimics. Nevertheless, novel prey with no traits in common with nonrewarding models were avoided at high rates, possibly in part due to their dissimilarity to familiar rewarding prey. The implications of these findings for mimicry evolution are discussed.