Complex Female Reproductive Tract Morphology: Its Possible Use in Postcopulatory Female Choice

Sperm competition has traditionally been modelled exclusively from the male perspective with females as passive participants. However females of many species have a physiologically and morphologically complex reproductive system (e.g. multiple sperm stores), which could enable them to control or influence offspring paternity by postcopulatory sperm selection. Previously the female reproductive tract has been modelled using a single sperm store. The model here describes the effects of a hostile site of insemination and two (or more) sperm stores in the outcome of sperm competition. If there was mortality of sperm directly after insemination, the main difference between having one or two stores was the number of possible choices. One-store females could choose the storage rates for different males and thereby influence paternity. Two-store females could choose the storage rates for different males differently in each store and later by choosing sperm from a particular store could favour a male at fertilisation. Without sperm mortality females with one store could hardly influence paternity. It was determined by female storage capacity relative to male ejaculate size. In contrast, females with two stores still achieved a clear separation of competing ejaculates, despite counteracting sperm displacement favouring last mates. This could give females the ultimate control over paternity through the selective use of sperm from each storage site, e.g. depending on environmental factors. As females of many animal groups may be able to spatially separate competing sperm in their multiple sperm stores (e.g. flies, spiders, birds) these results could have general significance for the evolution of female control of paternity.