Local mate competition modifies the costs of mating in a mostly monandrous parasitoid wasp

The costs and benefits of mating are frequently measured in order to understand why females mate multiply. However, to separate the factors that initiate the evolution of polyandry (from monandry) from the factors that maintain it, we must ascertain how the environment changes the economics of mating. Here we show how context-dependent costs of mating can lead to the evolution of polyandry in a species that is monandrous in the wild, the parasitoid wasp Nasonia vitripennis. We have previously shown that when females have insufficient time between mating and ovipositing, they appear unable to process sperm effectively and end up overproducing sons (i.e. laying unfertilised eggs, since Nasonia is in haplodiploid). This overproduction of sons is costly due to selection on sex allocation in this species. Although N. vitripennis is monandrous in the wild, polyandry evolves under laboratory culture despite this sex allocation cost. In this study we show why: when groups of females oviposit to...