The Effect of Season on Occurrence of Ovulation in the Rhesus Monkey1

Abstract

An experiment was designed to determine the frequency of the anovulatory cycles during summer and winter in rhesus monkeys housed under conditions of controlled light and temperature. The frequency of ovulatory menstrual cycles was determined for nine monkeys in the summer months of June, July, and August and again during December, January, and February. Occurrence of ovulation was determined by observations of the ovary at laparotomy. Five of twenty-four menstrual cycles during the summer and 30 of 31 cycles during the winter were ovulatory. However, the 5 ovulatory summer cycles were distributed among only two of the animals, the other seven animals did not ovulate during the summer. The proportion of monkeys which ovulated during the summer months was 22%. Summer menstrual cycles showed greater variability in length than those occurring during the winter. These data suggest that some monkeys maintain a seasonal anovulatory season, even in the absence of seasonal light and temperature variations. Other monkeys do ovulate during the summer, presenting the possibility of selecting these animals for breeding colonies.