DIMORPHISM: SEXUAL OR NATURAL SELECTION?

Debates over whether sex differences are maintained by natural or sexual selection go all the way back to Darwin and Wallace. These hypotheses are difficult to test, because we can't separate the ecological niches of the sexes from their reproductive behavior. Sex-limited polymorphisms are a unique tool for understanding the selective pressures on sexual dimorphism while controlling for sex. 

 

Female damselflies in Hawai'i exhibit striking color polymorphism--some are green, and others are orange or red, like males. My Masters work in the Cooper lab at James Madison University asked whether this polymorphism is maintained through density-dependent sexual conflict, with some females mimicking males to avoid mating harassment as in other damselfly species.
 

Two red damselflies copulating.

A red male and red female copulating. 

A red and a green damselfly copulating

A red male and green female copulating. 

Someone holds a red male, green female, and reddish female damselfly by the wings

A male, green female, and red female. 

Two woman stand in a pool in the rainforest. One hold a net, the other is dancing. Both are smiling

My behavioral study of courtship and mating behavior (pdf) showed that not only are there no density-dependent patterns of harassment, there is very little harassment at all. This is likely because males in this species, unlike the other damselfly species in which polymorphisms are studied, are territorial and do not aggressively pursue females outside of their territories. Instead of being under sexual selection, we believe that this polymorphism is an adaptation to divergent ecological selection in the niches used by males and females. 
 

Overall, this study points to two main lessons: First, that future studies of female polymorphism should test ecological as well as sexual selective forces; and second, that the same patterns of variation may be the result of different processes, especially when the species differ in ecology, natural history or mating system.