Sexual selection as the possible underlying force in calopterygid wing pigmentation: comparative evidence with Hetaerina and Calopteryx (Zygoptera: Calopterygidae)

Five hypotheses for the evolution of conspicuous 6 wing pigmentation have been proposed: sexual selection, differential niche utilisation, predator warning, social badge and ecological character displacement. Here, the sexual selection and ecological character displacement hypotheses are compared. First, the coefficients of variation (CVs) of pigmentation were compared against the CVs of a selected set of other animals' traits that are known to be maintained by either natural or sexual selection. Hetaerina americana, H. vulnerata, Calopteryx aequabilis, C. haemorrhoidalis and C. xanthostoma were used in order to compare CVs. Second, it was predicted that pigmentation should not differ in spp. whose populations are in sympatry (compared to allopatry) if sexual selection is driving the evolution of pigmentation (compared, for example, to an ecological character displacement hypothesis in which pigmentation between spp. should differ). Here, the pigmentation of sympatric and allopatric populations of H. americana and H. vulnerala were compared. The study produced 2 main results. First, the CVs of pigmentation were not different from the CVs of sexually selected traits in other animals; nevertheless, they were different from those of naturally selected traits. Second, the pigmentation of the 2 spp. in sympatry did not differ significantly. The same was true for allopatric populations. Taken together, these results suggest that sexual selection is the main mechanism of maintenance of pigmentation in these animals. Other alternative hypotheses for the evolution of pigmentation (differences in habitat use in both sexes, warning to predators by 6 6 and ecological character displacement) are discussed in the light of these results.