Female mating preferences and male signal variation in iridescent Hypolimnas butterflies

Nature's most striking, complex and innovative colour signals are generated via selective reflectance from optical nanostructures (rather than selective absorbance by pigments), a phenomenon known as structural coloration. These colours reach their height of visual brilliance as sexual signals, a context in which they also express both great functional innovation and high evolutionary lability. However, owing to a historical preoccupation with pigments, we know relatively little about the evolutionary causes and consequences of innovation in structurally coloured sexual signals, especially in exemplar arthropod taxa. In this study we addressed the possibility that species differences in intraspecific mating preferences may contribute to visual and functional variation in structural coloration. We contrasted mate preferences and signal properties between two closely related butterfly species (Hypolimnas alimena and Hypolimnas bolina) that possess male-elaborated structural coloration. Hypolimnas bolina offers a valuable comparative basis because females are known to prefer highly bright and limited-view ultraviolet markings, which males generate via complex nanoscale surface multilayer arrays. Male H. alimena, by contrast, display less bright and weakly iridescent dorsal blue, arising from a simpler surface microarchitecture. In two separate experiments, we found that female H. alimena did not distinguish between males spanning a graded range of 0.25-1.4× natural peak brightness. Only once the dorsal blue was completely obscured did male mating success suffer. Furthermore, a sample of wild phenotypes indicated greater variance for signal brightness in male H. bolina than H. alimena, but no difference in peak hue (i.e. signal colour). These results supported a priori predictions, and are consistent with a scenario whereby directional female preference has driven male signal exaggeration in H. bolina, but not its less ornamented close congener.