Unlearned responses of chicks, Gallus gallus domesticus, towards aposematic insect-like stimuli

Many predators avoid aposematic prey that possess conspicuous warning displays. This aversion can include innate and learned components, either of which could be exploited by profitable Batesian mimics that resemble aposematic prey. The perceptual bias hypothesis proposes that predators’ unlearned aversions may not correspond exactly with the patterns of aposematic prey, and that inaccurate Batesian mimics gain a fitness advantage by maximizing the unlearned aversion of their predators rather than their similarity to the model. This would provide a particular advantage when faced with naïve predators that have not yet had the opportunity to learn an association between the warning patterns and defences of the prey they encounter. We tested this hypothesis in the context of hoverflies that mimic stinging Hymenoptera: do naïve predators avoid wasp-like stimuli, and, if so, is this bias stronger when encountering inaccurate mimics? We presented domestic chicks, Gallus gallus domesticus , with arrays of 3D-printed stimuli each of which concealed a mealworm, Tenebrio molitor. Each array included control nonmimetic flies and alternative warning-coloured stimuli drawn from a set of wasps and honey bees (models), hoverflies (mimics) and artificially generated intermediates. The chicks had never encountered insect-like prey or stimuli (other than mealworms) prior to these trials. We found no difference in the order in which the stimuli were attacked in different treatments, but did find that chicks were slightly slower to open dishes with aposematic as opposed to control stimuli. Our results do not provide strong support for the perceptual bias hypothesis in the case of hoverflies, but we do find evidence of variation in the unlearned response towards insect stimuli dependent on appearance. These differences illustrate the importance of defensive signals, both honest and deceptive, in influencing predator behaviour even from the first encounter.