A Survey of Eyespot Sexual Dimorphism across Nymphalid Butterflies

Differences between sexes of the same species are widespread and are variable in nature. While it is often assumed that males are more ornamented than females, in the nymphalid butterfly genus Bicyclus, females have, on average, more eyespot wing color patterns than males. Here we extend these studies by surveying eyespot pattern sexual dimorphism across the Nymphalidae family of butterflies. Eyespot presence or absence was scored from a total of 38 wing compartments for two males and two females of each of 450 nymphalid species belonging to 399 different genera. Differences in eyespot number between sexes of each species were tallied for each wing surface (e.g., dorsal and ventral) of forewings and hindwings. In roughly 44% of the species with eyespots, females had more eyespots than males, in 34%, males had more eyespots than females, and, in the remaining 22% of the species, there was monomorphism in eyespot number. Dorsal and forewing surfaces were less patterned, but proportionally more dimorphic, than ventral and hindwing surfaces, respectively. In addition, wing compartments that frequently displayed eyespots were among the least sexually dimorphic. This survey suggests that dimorphism arises predominantly in “hidden” or “private” surfaces of a butterfly’s wing, as previously demonstrated for the genus Bicyclus. 1. Introduction Sexual dimorphisms are widespread and variable in nature and can result from either natural or sexual selection acting differentially on male and female traits [1–4]. Often, extensive mate-choice experiments, predation experiments, or documentation of sex-specific life histories need to be performed in single species before we understand which form of selection led to the evolution of the sex-specific traits [2]. A complementary approach to direct experimentation, however, is to survey the patterns of sexual dimorphism across a group of closely related organisms and discover whether these are congruent with the findings obtained for single members of the group. These comparative studies across species can help address whether the experimental knowledge obtained for a few species is generalizable across species. The eyespots in the nymphalid butterfly species Bicyclus anynana have been the subject of multiple laboratory experiments that concluded that they are likely evolving under both natural and sexual selection. Mate choice experiments as well as predation and mark-recapture experiments indicated that the dorsal eyespots are involved in mate signaling, whereas ventral eyespots play a role in deflecting the attacks of