Variation in female morph frequencies and mating frequencies: random, frequency-dependent harassment or male mimicry?

Abstract

Female-limited colour polymorphisms occur in a variety of species, where often one female morph (androchrome) resembles the body coloration of the conspecific male, whereas the other (gynochrome) does not. We tested predictions of two frequency-dependent hypotheses that are commonly invoked to explain the maintenance of these polymorphisms for multiple populations of the damselfly Ischnura elegans: (1) that males prefer mating with the most abundant female morph (LMR) or (2) that androchromes are functional male mimics (MM). We also asked whether variation in social and abiotic environments account for interpopulation variation in female morph frequencies. Contrary to predictions of the LMR hypothesis, morph mating frequency was not lower than expected based on morph frequency when a morph was uncommon or higher when a morph was most common. In support of the MM hypothesis, androchrome mating frequency was lower than predicted based on population morph frequency. In addition, as predicted, androchrome mating frequency increased with increasing ratio of androchromes to males, but we could not disentangle whether this was a consequence of the changing mimic/model ratio or because of rising androchrome frequency. Although variation in female morph frequencies between populations was not random across our study area, this could not be explained by geographical variation in frequencies and densities of males and females. Androchrome frequencies were higher in populations to the north and west where ambient temperatures were lower. Abiotic conditions such as temperature may need to be considered for understanding the maintenance of female-limited polymorphisms.