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We report on the energy expenditure and water flux, measured in the laboratory and in the field, of the Arabian oryx Oryx leucoryx, the largest desert ruminant for which measurements of the field metabolic rate of free-living individuals have been made using doubly labeled water. Prior to extirpation of this species in the wild in 1972, conservationists sequestered a number of individuals for captive breeding; in 1989, oryx were reintroduced in Saudi Arabia into Mahazat as-Sayd (2244km(2)). Apart from small pools of water available after rains, oryx do not have free-standing water available for drinking and therefore rely on grasses that they eat for preformed water intake as well as their energy needs. We tested whether oryx have a reduced fasting metabolic rate and total evaporative water loss (TEWL) in the laboratory, as do some other arid-adapted mammals, and whether oryx have high field metabolic rates (FMRs) and water influx rates (WIRs), as predicted by allometric equations for large arid-zone mammals. We measured FMR and WIR during the hot summer, when plant moisture content was low and ambient temperatures were high, and after winter rains, when the water content of grasses was high.

For captive oryx that weighed 84.1kg, fasting metabolic rate averaged 8980 kJ day(-1), 16.7% lower than predictions for Artiodactyla. Our own re-analysis of minimal metabolic rates among Artiodactyla yielded the equation: log(V) over dot O-2=-0.153+0.758logM, where (V) over dot O-2 is the rate of oxygen uptake in 1h(-1) and M is body mass in kg. Fasting metabolic rate of oryx was only 9.1% lower than predicted, suggesting that they do not have an unusually low metabolic rate. TEWL averaged 870.0 ml day(-1), 63.9% lower than predicted, a remarkably low value even compared with the camel, but the mechanisms that contribute to such low rates of water loss remain unresolved.

For free-living oryx, FMR was 11076 kJ day(-1) for animals with a mean body mass of 81.5 kg during summer, whereas it was 22081 kJ day(-1) for oryx in spring with a mean body mass of 89.0 kg, values that were 48.6% and 90.4% of allometric predictions, respectively. During summer, WIR averaged 1310ml H(2)Oday(-1), whereas in spring it was 3438 ml H2O day(-1). Compared with allometric predictions, WIR was 76.9% lower than expected in summer and 43.6% lower in spring. We found no evidence to support the view that the WIR of large desert ungulates is higher than that of their mesic counterparts. On the basis of the WIR of the oryx averaged over the year and the water contents of plants in their diet, we estimated that an oryx consumes 858kg of dry matter per year.