Female butterflies prefer males bearing bright iridescent ornamentation

Abstract

Butterflies are among nature's most colorful animals, and provide a living showcase for how extremely bright, chromatic and iridescent coloration can be generated by complex optical mechanisms. The gross characteristics of male butterfly colour patterns are understood to function for species and/or sex recognition, but it is not known whether female mate choice promotes visual exaggeration of this coloration. Here I show that females of the sexually dichromatic species Hypolimnas bolina prefer conspecific males that possess bright iridescent blue/ultraviolet dorsal ornamentation. In separate field and enclosure experiments, using both dramatic and graded wing colour manipulations, I demonstrate that a moderate qualitative reduction in signal brightness and chromaticity has the same consequences as removing the signal entirely. These findings validate a long-held hypothesis, and argue for the importance of intra- versus interspecific selection as the driving force behind the exaggeration of bright, iridescent butterfly colour patterns.

Figure 1

(a) Male Hypolimnas bolina, showing the ovoid dorsal colour patches which consist of a diffusely reflecting white spot overlaid with iridescent bluish-UV. (b) Scanning electron micrograph (21 400×) showing the tilted multilayering on the surface of a wing scale responsible for the male's iridescent coloration. (c) Reflectance spectra taken from the centre of the larger colour patch on two different forewings that were untreated (control), then manipulated by painting with rutin, or blacking out with pen (refer to §2). Forewings were from the same individual and reflectance is relative to an MgO standard; an Ocean Optics USB-2000 spectrometer was used with a pulsed PX-2 xenon light source.

Figure 2

The number of copulations achieved by control males (black columns) versus treatment males (grey columns) in each of the two enclosure-based mate choice experiments (‘pen’ and ‘rutin’). The visual nature of each manipulation is shown in figure 1c.