Female iridescent colour ornamentation in a butterfly that displays mutual ornamentation: is it a sexual signal?

In species wherein males display elaborate sexually selected traits or ornaments, conspecific females may also express these traits in full or partial form. This is called mutual ornamentation, and the co-occurrence of such traits in males and females remains poorly understood. In many sulphur butterflies (subfamily Coliadinae), males have a brilliant ultraviolet (UV) iridescence on their dorsal wing surfaces that functions as a sexual signal in courtship. In some of these sulphurs, such as the large grass yellow, Eurema hecabe, females also display dorsal iridescent patches, albeit smaller, restricted to the forewings and less bright than the male's, but the reasons for its occurrence in females are unknown. Here we present a study testing two functional hypotheses for the female UV-reflecting patch: an antiharassment hypothesis and a male mate choice hypothesis. The daily activity pattern of this species suggests that males are most likely to harass or choose among females from midday on. Observations made at this time of day on the characteristics of females related to male courtship duration suggest that males may preferentially court females with a large UV patch. Experiments with colour-manipulated models also suggest that males court with equal intensity females with and without a basal UV patch. Taken altogether the results provide no support for the antiharassment hypothesis. Because support for the male mate choice hypothesis was relatively weak and because of the limited potential for selection on female coloration due to male choice in these butterflies, the nonfunctional sexual correlation hypothesis remains a viable explanation for the female ornament.