164 CHARACTERIZATION OF THE COMMON ELAND (TAUROTRAGUS ORYX) ESTROUS CYCLE

Abstract

Three species of Tragelaphine (spiral horned) antelopes are endangered including the Eastern Bongo. Though most antelope species are represented as captive herds, a detailed characterization of their estrous cycles has not been described. In a crossover experimental design utilizing two groups of four females (n = 8), two estrus synchronization treatments were applied to each female. The treatments were: 1) PGF_2∝ (Lutalyse^®, 25 mg IM) repeated 11 days later and 2) administration of a progestin (altrenogest, 5 mL, 2.2% p.o.) for 7 days followed by PGF_2∝ (25 mg IM) on 7th day. Blood samples were collected three days (Monday, Wednesday and Friday) per week until 2 days before expected estrus when ovaries were examined via transrectal ultrasound once daily and blood samples were collected twice daily, until the dominant follicle was absent. Blood samples were then collected three days per week until two days before the predicted subsequent natural estrus, when intensive ultrasound and blood collection was resumed. Plasma progesterone levels were determined via competitive radioimmunoassay utilizing a double antibody system from a commercially available kit (Diagnostic Systems Laboratories, Webster, TX, USA). Progesterone levels declined in an equal number of females (n = 6 or 75%) following prostaglandin injection in treatments 1 and 2, suggesting both methods to synchronize the estrous cycle or induce estrus are equally effective. Five of the eight females ovulated at least once during the study. A total of eight (25%) of the 32 possible ovulations were detected by ultrasound observation and confirmed by progesterone levels. Basal progesterone levels for the ovulating females around the days of ovulation were approximately 0.5 ng mL^–1 and peaked at approximately 2 ng mL^–1 during the luteal phase. Ultrasound observed ovulations occurred as frequently on the right ovary as the left. Two animals from treatment 2 ovulated during both the induced and natural cycles on alternate ovaries. An interovulatory interval of 21 days was observed and agrees with data previously reported by Durrant (1983 Zoo Biol. 2, 191–197). The average time to ovulation after PGF_2∝ injection was 69 h (range 58–82 h). The average follicular size 12 to 24 h prior to ovulation was 9 and 7 mm for the induced and natural ovulations, respectively, suggesting that eland follicles ovulate at 10 mm. The described technology allows hormonal and ultrasonographic characterization of the estrous cycle in the eland antelope.