Menstrual cycle characteristics of seasonally breeding rhesus monkeys

Rhesus monkeys in seminatural environments exhibit a distinct seasonal mating cycle with conceptions restricted to the fall and winter months. In the present study, the characteristics of menstrual cycles were examined during a 1-year period in twelve rhesus monkeys in whom pregnancy was prevented. Menses occurred throughout the year, but ovulations were observed only in the fall and winter months. Menses in the spring and summer months occurred irregularly and were associated exclusively with anovulatory cycles. The total number of ovulations exhibited by these females during the breeding season ranged from two to six and was positively related to body weight, mean luteal phase progesterone (P) levels of normal cycles and social dominance rank. Ovulations with a short luteal phase were exhibited by four females (seven cycles), with the likelihood of occurrence increasing as the breeding season progressed. The incidence of abnormal cycles was predicted from the linear combination of parity, body weight and luteal phase P of normal ovulatory cycles. These results suggest that during the seasonally delimited period of ovulation, females exhibit a range in the quality and quantity of ovulations which may be predicted by certain idiosyncratic physical and behavioral traits.