Plasma Gonadotropins, Prolactin, Gonadal Steroids, and Genital Swelling during the Menstrual Cycle of Lowland Gorillas*

Plasma LH, FSH, PRL, 17β-estradiol, progesterone, and testosterone were measured during four menstrual cycles in three gorillas, and their relationship to genital swelling (labial tumescence) was assessed as a contribution to the comparative perspective on primate reproductive physiology. Cycle length varied considerably (23–48 days), but the length of the luteal phase was relatively constant (11–13 days). All cycles exhibited a midcycle LH peak (25.9–48.1 mIU/ml) and a luteal phase elevation in progesterone (10.8–12.2 ng/ml), suggesting normal ovulatory function. FSH increased at midcycle but not as consistently as LH. PRL showed no rhythmicity. 17β-Estradiol showed a midcycle elevation (200–500 pg/ml) that preceded the LH peak and a luteal phase elevation that approximated the time course of progesterone. Progesterone was barely detectable during the follicular phase (<0.5 ng/ml), rose on the day of the LH peak (n = 2) or a day later (n = 2), peaked 6–7 days after the LH peak, and decreased to low levels at menses. Testosterone exhibited a midcycle peak of 350–500 pg/ml compared to follicular and luteal phase values (150–300 pg/ml). Labial tumescence was minimal at menses (1–2 cm), rose after 6–16 days, and reached maximum (3–4 cm) 9–35 days after menses on the day before the LH peak (n = 4) in association with the midcycle elevation in 17β-estradiol. Detumescence occurred 0–3 days after the LH peak, when l7β-estradiol was decreasing and progesterone was increasing. The gorilla shares with the chimpanzee a somewhat closer relationship to man than do the monkeys with respect to certain aspects of the menstrual cycle hormone patterns studied.