Timing of insemination is correlated with the secondary sex ratio of Norway rats.

Timing of insemination in a seminatural environment had a significant, curvilinear relationship with the secondary sex ratio of litters conceived during cycling estrus. When fifth inseminations were received prior to 2.75 h after dark onset, there was a steady decline in the sex ratio as time between dark onset and fifth insemination increased. Fifth inseminations received after 2.75 h resulted in a moderate rise in the sex ratio. This curvilinear relationship was also found with artificial insemination, indicating that neither copulatory behavior nor the vaginal environment were necessary for the association. For postpartum litters there was a linear decline in sex ratio as the interval between dark onset and fifth insemination increased; however, there was no subsequent rise in sex ratio with late inseminations. Season of conception and parity of the dam were associated with sex ratio; litter size and estrous condition were not. Feasibility of the rat as an animal model for sex determination in humans is discussed.