Variation in Ovarian Steroids Associated with the Annual Mating Period in Female Rhesus Monkeys (Macaca mulatta)1,2

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Abstract
Female rhesus monkeys living in naturalistic environments exhibit seasonal breeding with copulations typically restricted to the period associated with ovulation and conception. In order to determine how the occurrence of a female’s mating period and the rate of copulations within the period were influenced by specific endocrine events, the sexual behavior of 16 female rhesus monkeys, living in a heterosexual group in an outdoor compound, was monitored daily. Serum samples. collected three times weekly during the early follicular and luteal periods and daily during the late follicular and periovulatory periods, were analyzed for estradiol (E2), progesterone (P), and testosterone (T). Females exhibited a discrete period of copulatory behavior (16.5 ± 2.1 days) associated with first ovulation of the breeding season. Copulatory activity was first observed following significant increases in serum E2 levels and ceased when serum P levels rose significantly during the luteal phase. Neither the onset nor termination of the mating period was associated with significant changes in serum T. The occurrence of the female mating period was positively correlated with E2 and negatively correlated with P; moreover, the statistical combination of E2 and P accounted for an even greater proportion of the total variance. Thus, a female’s mating period was associated with elevated levels of E2 (>90 pg/ml) in the presence of low levels of P (2 levels were low (3.00 ng/ml) regardless of the concentration of E2. The rate of copulations within a female’s mating period was not related to daily fluctuations in serum E2, P, or T; however, a significantly higher rate of female copulatory behavior was observed during the 1- to 4-day interval following the E2 peak, when P levels were intermediate (1.50 ng/ml). These data suggest that P may synergize with E2 to facilitate the rate of rhesus female copulatory behavior during the period when fertilization is most probable.