DURATION OF THE ESTROUS-CYCLE AND CHANGES IN PLASMA-HORMONE CONCENTRATIONS MEASURED AFTER AN INDUCED OVULATION IN SCIMITAR-HORNED ORYX (ORYX DAMMAH)

To determine the duration and hormonal characteristics of the oestrous cycle in scimitar-horned oryx, ovulation was synchronized in 11 femal es using exogenous progesterone and a prostaglandin analogue. Daily bl ood samples were taken from six animals during 46 days (with a 7-day h iatus). The remaining five animals were blood sampled every 30 min for 6 h on three occasions during 22 days, then resynchronized with prost aglandin analogue and pregnant mare's serum gonadotrophin and blood sa mpled daily for another 21 days (with a 7-day hiatus). Behavioural obs ervations were made on days 1 through 3 and 24 through 28 to determine the occurrence of oestrus. Only animals > 2 years old (n = 7) had pla sma progesterone concentrations indicative of luteal function, with a mean luteal phase progesterone concentration of 1.36+/-3.3 (S.E.) nmol .l(-1) and a mean follicular phase concentration of < 32 nmol.l(-1). M ean duration of the oestrous cycle estimated from plasma progesterone profiles and behavioural data 22.7+/-1.1 (S.E.; n = 7) days, range 17- 25 days. When estimated from behavioural data alone, mean duration of the oestrous cycle was 22.3+/-0.3 (S.E.; n = 3) days, range 22-23 days . Mean length of the luteal phase was 17.3+/-0.6 (S.E.; n = 8) days, r ange 14-19 days, and mean length of the follicular phase was 5.6+/-0.5 (S.E.: n = 5) days, range 3-7 days. Pulses in plasma LH concentration detected by Cluster analysis ranged from 0.34+/-1.55 ng.ml(-1) in ani mals > 2 years old (n = 3), and from 0.17-0.29 ng.ml(-1) in animals < 2 years old (n = 2), the difference in mean pulse height being signifi cant (P < 0.01). No difference in mean plasma LH concentration was det ected due to stage of cycle. Elevations in plasma oestradiol-17 beta c oncentration were observed at or preceding oestrus in three animals an d during the mid-luteal phase in three animals. A significant decrease in haematocrit with time was recorded for blood samples From the thir d serial bleed. This was greater (P < 0.01) in animals > 2 years of ag e (n = 3), suggesting that these older animals adapted better to the s tress of repeated sampling. Plasma cortisol concentrations also declin ed throughout the serial bleed but there was no difference in this dec line between the two age groups. Results of this study define the dura tion of the oestrous cycle in scimitar-horned oryx and show that its h ormonal components are similar to the well-characterized patterns of o ther species, especially cattle.