Maturational delay of young female mice as the result of exposure to grouped female odors and reproductive inhibition as the result of exposure to isolated adult females have both been observed (Drickamer, 1974; Skryja, 1978). Each has the potential to reduce the growth rate of populations. Reductions in a female's reproduction facilitated by social stimulation from other females, while effective in reducing population growth, may in the case, of maturational delay and reproductive inhibition be an epiphenomenon or exadaptation of selection for improved relative reproductive success in the females possessing these abilities. The ultimate outcome of these selective processes may be the buffering of population numbers, but the selective forces may operate to maximize a female's relative reproductive success. A females' relative reproductive success can be maximized by either increasing her own reproduction or decreasing the reproductive output of other females. A body of evidence exists to suggest that in Peromyscus mamculatus and Peromyscus leucopus, females are physiologically constrained and unable to increase their own reproduction. If the assumption of physiological restraint is correct, then the most effective way for females to maximize their relative reproductive success is to reduce the reproductive output of their competitors. In this paper, maturational delay and reproductive inhibition as they affect both the adult female and young females are discussed. Examination of these effects reveals that while they can be effective in population regulation, their main function and the selective process that produced them is at the level of individual reproduction.