Timing of mating, developmental asynchrony and the sex ratio in mice.

According to the developmental asynchrony hypothesis, changing the time of mating within the estrous cycle could alter the interval between completion of blastocyst development and uterine responsiveness for implantation. This may then lead to sex ratio skews in animals that exhibit sex-differential blastocyst development, because uterine stage may now benefit either slow (female) or fast (male) developing blastocysts. To test this hypothesis, the responses of two strains of mice to altered mating dynamics were compared. In a strain that exhibits higher male than female blastocyst developmental rates, sex ratios became significantly female-biased when mated late during the estrous cycle as opposed to early mating. However, timing of mating did not affect sex ratios in a strain with synchronous development of male and female preimplantation embryos. Hence, it is concluded that developmental asynchrony between male and female blastocysts on the one hand, and blastocysts and uterus on the other, are indeed responsible for the effect of timing of mating on litter sex ratios in mice.