CYCLIC PATTERNS OF LUTEINIZING HORMONE, FOLLICLE-STIMULATING HORMONE, INHIBIN, AND PROGESTERONE SECRETION IN THE ASIAN ELEPHANT {ELEPHAS MAXIMUS)

Serum samples were collected one to three times weekly from four unanesthetized Asian elephants (Elephas maximus) for 6-18 consecutive months. Based on circulating progesterone pro files, 14 complete ovarian cycles were observed. The estrous cycle averaged 13.2 ? 0.7 wk in length, with an active luteal phase of 9.8 ? 0.7 wk. Increases in serum luteinizing hormone (LH) were observed immediately before or during the progesterone rise in 11 of 14 cycles. In eight cycles, a second LH surge was detected 11-19 days later. Radioimmunoassays for follicle-stimulating hor mone (FSH) and inhibin were validated for elephant serum. Fluctuations in FSH and inhibin secretion also were observed at 12-14-wk intervals, although their temporal profiles differed from each other and from that of progesterone. Follicle-stimulating hormone concentrations were lowest during the late follicular and early luteal phases and then increased to peak levels during the latter part of the luteal phase. In contrast, serum inhibin concentrations were inversely related to FSH levels throughout the estrous cycle (r = -0.78; P < 0.01). In summary, progesterone analyses confirm that the luteal phase in the Asian elephant is approximately 10 wk long. Furthermore, the 12-14-wk oscillations in serum FSH and inhibin secretion provide additional evidence that the ovarian cycle of this species is several months in duration. The inverse relationship between serum FSH and inhibin suggests that inhibin may regulate FSH secretion, as is described for other species. Elevated FSH secretion throughout the mid- and late luteal phase may stimulate waves of follicular growth that are responsible for the short "follicular cycles" described in earlier reports.