Age differences in Copulatory behavior and serum 17 beta-estradiol in female rhesus monkeys

Young, sexually mature female rhesus monkeys housed in outdoor groups have been found to exhibit significantly longer mating periods associated with first conception compared with the duration of the annual period exhibited by older, multiparous females. An analysis of the sexual behavior and corresponding serum concentration of 17 beta-estradiol (E2) a progesterone, (P) in two age groups of rhesus females revealed that young females copulated on significantly more days prior to the E2, ovulatory peak than did older females. Serum concentrations of E2 were significantly higher from -30 to Day- 10 prior to the E2 peak (Day 0) in young females. Estradiol concentrations were similar for both age groups during the remaining follicular and periovulatory period. No age differences in serum P were observed between the two groups. Serum E2 levels associated with the onset of mating were similar in the two age groups with levels rising from 73.5 +/- 4.0 pg/ml prior to mating to 97.2 +/- 4.6 pg/ml. Thus the hormonal profile associated with the onset of mating is similar for both age groups, and the earlier occurrence of mating behavior prior to ovulation in young females is accounted for by an earlier rise in serum E2.