Timing of sexual maturity in female rhesus monkeys (Macaca mulatta) housed outdoors

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Abstract
A comparison of the age and season at 1st parturition was made for spring-born female rhesus monkeys and for females born in the fall to mothers who had been laboratory-housed before being transferred outdoors. Females (n = 9) born during the fall had 1st parturition during the spring and summer, as did all spring-born females (n = 68), and not during the fall as would be predicted if age were the determining factor. A separate analysis of post-menarchial, spring-born females (n = 5) beginning in Sept. at 29 mo. of age revealed that the ensuing 12 mo. were characterized by low serum levels of estradiol (< 50 pg/ml), progesterone (< 1.0 ng/ml), LH [luteinizing hormone] (< 7.0 ng/ml), and FSH (< 5.50 ng/ml). First ovulation subsequently occurred in the fall in all subjects at a mean age of 41.9 .+-. 0.1 mo., and was preceded by significant elevations in basal LH and FSH, coincident in time with the transition of summer to fall (Sept.). Female copulatory behavior was restricted to the period surrounding 1st ovulation, beginning some 2 wk before and ceasing within 3 days after the estradiol peak. The most rapid gain in weight occurred during the summer months before 1st ovulation, and was associated with significant elevations in serum GH [growth hormone] and prolactin. Season may influence the timing of sexual maturation in rhesus monkeys kept outside in such a way that the occurrence of 1st ovulation is restricted to the fall and winter months.