Why do Females Make it so Difficult for Males to Fertilize their Eggs?

In terms of its structure, chemical composition and immune response, the female reproductive tract of mammals and birds is particularly hostile to sperm. Only a tiny proportion of the sperm transferred during copulation ever reach the ova. We consider a number of explanations for why female anti-sperm responses have evolved in birds and mammals, including avoidance of the following: infection, unfit sperm and polyspermy. We present a new hypothesis which proposes that anti-sperm responses allow females, through sexual selection, to exercise choice over which male or which sperm fertilize their eggs. We propose that females which are constrained in their initial choice of partner copulate with more than one male during a single reproductive cycle and allow the sperm from the different males to compete for fertilizations. This could work in two ways: either females exploit the hostile nature of their reproductive tract, which evolved for other reasons, to select sperm. Or, the hostility of the female tract has evolved, in part at least, to "test" sperm. If females have evolved more and more effective barriers to sperm in their reproductive tract as means of mate choice, coevolution between female anti-sperm responses and male abilities to overcome these will result in increasingly elaborate forms of female hostility towards sperm.