Fecal and urinary progestin analyses have shown that giraffes express a short reproductive cycle, averaging 15 days, compared with other large ruminants. However, actual ovarian events have not been correlated with the hormonal pattern. In this study, mature cycling female Rothschild giraffes (Giraffa camelopardalis rothschildi) were repeatedly examined by transrectal ultrasonography to correlate ovarian function with changes in fecal progestin (fP4 [nc = 6]) and estradiol (fE2 [nc = 6]) and serum progestin (nc=2) as measured by enzyme immunoassay. Five females became pregnant and were monitored during early gestation. In this study, we discovered that hormone values for fP4 in cycling giraffes do not correlate with the classic profile of follicular development, ovulation, and luteogenesis. The corpus luteum (CL) and the next dominant follicle were forming simultaneously. A mean 6 SD peak in fE2 of 254.92 6 194.76 ng/g and subsequent ovulation occurred as early as 1 day after the fall in fP4. In pregnant giraffes, the CL reached a diameter significantly larger (mean 6 SD, 41.02 6 2.70 mm; P=0.0126) than that during the cycle (33.48 6 2.80 mm), while follicular activity and fluctuating fE2 were still present. With this research, we demonstrated that the progesterone profile typically used to characterize the ovarian cycle does not correlate with luteal development in the ovaries of this species. Furthermore, we conclude that the giraffe could have evolved a short reproductive cycle because of the almost parallel order of ovarian events.
