Non-warning odors trigger innate color aversions—as long as they are novel

Warning signals made by unpalatable insects to potential predators commonly target more than one sense: such signals are ‘‘multimodal.’’ Pyrazines are odors produced by warningly colored insects when attacked, and have been shown to interact with food coloration, biasing avian predators against novel and typically aposematic food. However, at present it is not known whether this is an adaptation by prey to exploit a general feature of avian psychology, or an evolutionary response by birds to enhance their avoidance of unpalatable prey. Here we investigate the effect of other odors on the innate responses of naive domestic chicks (Gallus gallus domesticus) to food that is of novel color, or of a color that is associated with warning coloration, yellow. In the first experiment, we demonstrate that natural and artificial odors that have no association with aposematism in the wild can produce biases against both novel colored foods and yellow colored foods. In a second experiment, we also show that odor novelty is vital for eliciting such effects. These results support the idea that warning odors have evolved in response to preexisting psychological biases against novel odors in predators, rather than predators evolving specific responses against odors associated with unpalatable prey.