Spectral variation and pigmentary basis of ornamental and mimetic wing colour patches of swallowtail butterflies

Colours and colour patterns are extraordinarily diverse traits that are often used as visual signals. To test ecological and evolutionary drivers of these visual signals, a clear understanding of their nature and variation is necessary. Here we characterise variation in wing colouration of Asian mormon swallowtail butterflies ( Papilio , subgenus Menelaides ). These species exhibit two kinds of colour patches on largely black wings: creamy white/yellow/green patches that are presumably used as sexual ornaments, and pure white patches that are presumably used as mimetic signals. Using reflectance spectrophotometry we quantified spectral properties of black wing background and colour patches between sexes, wing surfaces and mimicry status. We discovered that brightness and saturation of the black background were less variable across sexes, wing surfaces and mimetic/non-mimetic status. However, colour contrast and saturation were higher on dorsal surfaces than on ventral surfaces, and colour contrast between the black background and colour patches was higher in males than in females. Colour patches in non-mimetic butterflies were brighter and more saturated across the whole colour spectrum compared to mimetic butterflies. These patterns of colour variations in relation to their putative functions suggest that: (a) colour patches on dorsal and ventral wing surfaces evolve independently to accommodate differential strengths of natural and sexual selection, (b) sexual ornaments are brighter in non-mimetic males but they also occur in all non-mimetic females, indicating mutual sexual selection on these ornaments, but which is stronger in males, and (c) mimetic male and female butterflies display less sexual contrast in colour patches, indicating relatively similar strength of mimetic selection. Thus, our study characterises colour variation in an extraordinary signal radiation on the wings of swallowtail butterflies, a model clade in ecology, evolution and genetics. Finally, using liquid chromatography-mass spectrometry (LCMS) we identified the pigment papiliochrome-II to be the chemical basis of the presumed sexual ornaments in mormon swallowtails.