In male house mice (Mus domesticus and M. musculus), the act of coital ejaculation provides a fail-safe neural signal for timing the birth of their offspring. A unique aspect of this phenomenon is the extraordinary latency that can occur between the stimulus (ejaculation) and its adaptive neural response (male mice cease killing pups and behave parentally toward them). Thus the inhibition of infanticide is routinely time-delayed for many days after mating. In the absence of mating, cohabitation with a female will not inhibit infanticide in CF-1 stock males (M. domesticus), whereas the birth of pups in the male's home cage will inhibit infanticide. But with regard to the ejaculatory phenomenon, which also includes the spontaneous reemergence of infanticide 50-60 days after mating, this entire behavioral cycle toward pups can occur in the total absence of regular time cues from a light/dark cycle following ejaculation. However, exposure to photoperiodic (L:D 12:12) or constant light (LL) accelerated the transition time from infanticide to parenting after ejaculation, while in constant dark (DD), the transition time to parenting was significantly prolonged. The time interval between ejaculation and the inhibition of infanticide, which varied among individuals first mated at 6 months of age, was repeatable when the same males were remated at 9 months of age; however, when males were again mated at 18 months of age, the time interval between ejaculation and parenting was dramatically prolonged. In general, coital ejaculation triggers a neural timing system that cannot be explained by any presently known physiological mechanism. Our results do suggest, however, that the neural timing variation observed among individuals is influenced by sex steroid exposure during late fetal development.