Heightened phenotypic variation and age-based fading of ultraviolet butterfly wing coloration

Hypothesis: Structural coloration (iridescent ornamentation) should vary more greatly among individuals and with age than pigment coloration. Structural coloration requires nano-scale precision and might thus more honestly signal male age and condition than pigment-based colours. Organism: A natural population of the orange sulphur butterfly, Colias eurytheme. C. eurytheme females prefer young males, choosing them on the basis of an unknown signal. Site: Cultivated alfalfa (Medicago sativa) fields near Avondale, Arizona, August-September 2004. Methods: I randomly sampled 70 'pristine' males and an additional 98 males in five age classes. I then measured key reflectance characteristics (brightness, hue and chroma) of the dorsal structural ultraviolet markings and of the coincident pigmentary yellowish-orange. Conclusions: Consistent with the sexual selection hypothesis, structural reflectance parameters are more phenotypically variable. Structural reflectance also fades more with age. However, the higher phenotypic variance of structural colour renders it a less precise indicator of male age than pigment colour.