Effects of rainfall on the reproduction and mortality of the Arabian oryx Oryx leucoryx (Pallas) in the Sultanate of Oman
The effects of rainfall on the reproduction and mortality of the Arabian oryx Oryx leucoryx reintroduced in Oman, were investigated for the years 1987 to 1993. In order to examine any relationship between reproduction or mortality and a pattern of rainfall that is neither temporally nor spatially predictable the effect of rainfall on the nutritional quality of the forage species of the oryx was examined. The crude protein content of the primary feed species of the oryx declined exponentially from a maximum following rainfall, to 6% after about 200 days and thereafter remained above 5% for up to three years. Maintenance requirements of 5%-6% were estimated for the oryx. Utilising the relationship between rainfall and crude protein an index of dietary crude protein was developed. The index was used to estimate the crude protein in the diet of an oryx at any time and these estimates were related to reproduction and mortality. Oryx calved in each month of the year and in desert born females births peaked at times of low estimated dietary crude protein whilst conception peaks followed good nutritional conditions. Consequently, birth dates were merely the result of nutritional factors acting on conception. The proportion of desert born animals that conceived in any month increased with increasing estimated dietary crude protein in that month and the preceding two months. On average oryx calved for the first time at 28.2 5.3 months although in the best conditions they calved at 20.7 months. Nutritional conditions at maturity (12-15 months of age), as measured by estimated dietary crude protein, were found to influence the age at first breeding. Inter-calf intervals averaged 11.9 3.5 months and interval length was influenced by estimated crude protein in the mother's diet 3-6 months before conception and by mother's cohort. Sex ratio at birth and sex ratio at weaning did not differ from the expected 50:50. The probability of an oryx being fecund in any year of its life decreased at low levels of crude protein in the preceding year of its life.