A within-species warning function for an aposematic signal.

Aposematic, or warning, signals are generally interspecific in form: one species advertises noxiousness to a predator or parasite species. In a study of the pipevine swallowtail butterfly (Battus philenor), we show that a pattern of colouration in the caterpillars that is considered to be aposematic in the context of attack by natural enemies also deters oviposition by conspecific females. In field and laboratory assays, females avoided oviposition on plants bearing live conspecific larvae. Females avoided oviposition on plants bearing artificially constructed models identical to larvae in shape, size and colour pattern. Finally, oviposition on plants harbouring a model bearing the larval colour pattern was reduced relative to plants bearing a leaf-green model, suggesting that the larval colour pattern was essential for avoidance. We discuss how intraspecific and interspecific processes might interact in the evolution of an aposematic signal.