Female mating biases for bright ultraviolet iridescence in the butterfly Eurema hecabe (Pieridae)

Abstract

Exaggerated male-limited coloration is widespread among butterflies, yet convincing demonstrations of intraspecific mating preferences for signal brightness and/or chromaticity are relatively rare in this group. Here, I couple behavioral experiments involving manipulations of ambient light environments and male reflectance patterns with observation of wild mating patterns to investigate visual mating biases in the large grass yellow (Eurema hecabe). Males in this species possess exaggerated, limited-view ultraviolet (UV) iridescence across most of their dorsal wing surface that has putative sexual signaling function. In the first experiment, conducted in small (0.7–m3) cages, individuals were significantly less likely to copulate when the UV portion of natural ambient illumination (i.e., 300–400 nm) was strongly reduced. In 2 subsequent experiments, conducted under full-spectrum sunlight in small and large (5 × 6 × 4 m) cages, males with their UV signal artificially dulled by 25% consistently copulated with fewer, and smaller, females than sham-control individuals. Importantly, the manipulated levels of UV brightness in these experiments fall well within the naturally occurring bounds of variation in male UV reflectance. These findings therefore unanimously support the presence of a UV signal–based female bias. In apparent contrast, comparison of 161 in-copula and 188 free-flying males from a high-density field assemblage revealed that copulating males were significantly older and henceforth actually possessed (subtly) less UV bright wings. Copulating male UV brightness was, however, positively related to the size of their mate, which echoes the experimental findings and may represent a signature of mutual mate choice. I discuss these results in light of the full complexities of the butterfly mating system and the potential signaling value of iridescent coloration in butterflies and animals generally.