Estrous Cyclicity in Mice Housed in the Presence or Absence of Males

The majority of CD∗-1 female mice housed individually with adult males exhibit 4-day estrous cycles. Regular cyclicity is observed in these animals from the time daily vaginal smear examinations are begun. The majority of mice housed individually without males exhibit 5-day cycles, but frequently only after periods of non-cyclicity following the start of vaginal smear examinations.