A description of the oestrous cycle of scimitar-horned oryx, Oryx dammah

Abstract

To determine the duration and hormonal characteristics of the oestrous cycle, ovulation was synchronized in 11 female scimitar-homed oryx using exogenous progesterone and a prostaglandin F₂α analogue. Daily blood samples were taken from 6 of the animals over 46 days (with a 7 day hiatus). The remaining 5 animals were blood sampled every 30 minutes for 6 hours on 3 occasions over 22 days, then resynchronized with prostaglandin F₂α analogue and pregnant mare serum gonadotrophin and blood sampled daily for another 21 days (with a 7 day hiatus). Progesterone and oestradiol-17β concentrations were measured in daily blood samples and plasma ovine-luteinizing-hormone-like immunoreactivity (LH) was assayed in serial samples. Haematocrits were determined for blood samples from the third serial bleed. Duration of the oestrous cycle estimated from hormonal and behavioural data was 22.7 ± 1.1 (SEM; n=7) days, range 17-25 days. Duration of the oestrous cycle estimated from behavioural data alone was 22.33 ± 0.33 (SEM; n=3) days, range 22-23 days. Mean length of the luteal phase was 17.3 ± 0.6 (SEM; n=8) days, range 14-19 days, and mean length of the follicular phase was 5.6 ± 0.5 (SEM; n=5) days, range 3- 7 days. Only animals >2 years old (n=7) showed plasma progesterone concentrations indicative of luteal function, with a mean luteal phase progesterone concentration of 13.6 ± 3.3 (SEM) nmol.l⁻¹ and a mean follicular phase concentration of <3.2 nmol.l⁻¹. Episodic changes in plasma LH concentrations were detected by visual inspection and Cluster analysis. Pulses detected by Cluster analysis ranged from 0.45-1.15 ng.ml⁻¹ in animals >2 years old (n=3), while the sole pulse detected in a younger animal was only 0.26 ng.ml⁻¹. There was no difference in mean plasma LH concentration due to stage of cycle. Elevations in plasma oestradiol-17β concentration were observed at or preceding oestrus in 3 animals and during midluteal phase in 3 animals. A significant decrease in haematocrit over time was recorded for blood samples from the third serial bleed. This was greater (P<0.01) in animals over 2 years of age (n=3) which indicates handling stress as a possible factor in the apparent lack of ovarian funtion in the younger animals. This study provides the first substantial description of the duration of the oestrous cycle and the pattern of progesterone secretion in the scimitar-horned oryx. The other hormonal results, while inconclusive, indicate the possibility of a midluteal wave of follicular development and of pulsatile secretion of LH.