Selection for multiple mating in females due to mates that reduce female fitness

If females are unable to discriminate among males before mating, remating by females that store sperm may have evolved as a hedge against having only "costly" mates (less preferred males that reduce her fitness). However, the benefit of remating is not guaranteed because she can also mate by chance with another costly male. We devised a model to explain the evolution of female remating by representing female fitness as a function of the proportion of costly mates. We examined the effect of a linear, a concave-up, and a concave-down fitness function and found that only the latter favors the evolution of female remating. With a concave-down function, females mating with 50% costly mates have nearly the same fitness as do females with none. A biological interpretation for a concave-down function is that sperm from good males are better at competing with sperm from costly males or are more preferred by females. A concave-up function implies the reverse, whereas a linear function will occur when sperm are equally competitive. We review specific situations in nature that might produce a concave-down function and find evidence that sterility and intragenomic conflict are two phenomena capable of driving the evolution of female remating by our model. Copyright 2003.