Under these circumstances, females are expected to evolve mechanisms to control exposure to the male ejaculate in order to maximize fitness by balancing the positive and negative impacts of SFP. This cost to the female is the basis of sexual conflict in Drosophila, an evolutionary arms race between the sexes. SFP can however compromise female fitness and even shorten her life-span. Other Drosophila SFP also confer a competitive advantage to males by altering female behavior (e.g. melanogaster, males use SFP to construct a mating plug in the female reproductive tract to block insemination by other males. The reproductive success of males, on the other hand, will often depend upon avoidance of sperm competition by preventing mated females from copulating and receiving sperm from other male suitors. A remarkable example of CFC was demonstrated in the domestic fowl where sperm from socially subordinate males is preferentially ejected from the female reproductive tract (henceforth, sperm ejection). It is now recognized that both of these examples of PCSS can have profound biological impact in driving evolutionary change. These polyandrous mating systems can increase female fitness by reducing the risk of infertility and, when sperm from different males co-reside in the female, can provide opportunities for competition between sperm from different ejaculates and for females to bias paternity towards the sperm of a preferred male, a phenomenon known as CFC. It is common place in the Animal Kingdom for females to mate with more than one male.
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