Male influence on estrous cycle phenomena in the rat

Sixty-four colony-bred Sprague-Dawley female rats were used in examining the effect of the male presence on female estrous cycle interval. Females were randomly assigned 4 per cage to 16 cages. Vaginal smears were taken daily during a 28-day period. Detection of cornified cells was assumed to be associated with estrus. There was considerable variability in Individual estrous cycle length. The mean number of cycles exhibited was 3.84 during the 28-day premale period. The expected mean number of cycles would have been 4.7 to 5.4 if all females had exhibited regular estrous cycles of 5 to 6 days duration. The average length of estrous interval prior to exposure to males was 5.18 days. On day 28 a mature intact male was introduced within each pen. Prior to introduction, males were housed separately from females, mixing of colony odors was minimized by room ventilation. Males were removed from females 17 days following introduction. Following introduction of males, an average estrous interval length of 5.28 days was observed. This did not differ significantly from the 5.18-day average interval observed prior to exposure to males. Thus, no significant male influence upon estrous interval length was demonstrated. One-hundred percent of the females, including those showing abnormal estrous patterns prior to the introduction of the males, showed cornified cells within the first 5 days following introduction of the male and 76 percent of this number conceived within this first 5-day period. The difference between percent showing cornified cells and the percent conceiving suggests that observation of cornified cells may not be a reliable indicator of sexual receptivity in the rat.