It has long been thought that some female mammals (e.g., humans and Norway rats) synchronize their ovarian cycles when in close proximity. However, re-analyses of these studies have revealed serious and systematic methodological errors. In retrospect, this is exactly what should have been expected given that female mammals exhibit considerable ovarian cycle-length variability, which prevents synchrony. Despite the diversity of cycle lengths in mammals, little attention has been paid to possible adaptive functions of cycle-length in female mate choice. Thus an individual-based model of Norway rats is described here that uses actual data from an experimentally constructed habitat. Results showed that synchrony is costly to females in achieving matings with high quality males because synchrony forces female-female competition. Synchrony can be avoided by evolving longer cycles, but there are limits on cycle length due to the cost of waiting. Furthermore, it is possible for inter- and intra-female cycle length variability to be evolutionarily stable within a population; thus offering a plausible explanation for some of the diversity observed in ovarian cycle lengths in female mammals.