SUMMARY To test the idea that large, free-living, desert ungulates use heterothermy to reduce water loss, we measured core body temperature (T_b) of six free-ranging, adult Arabian oryx (Oryx leucoryx) during 2 years in the arid desert of west-central Saudi Arabia. We report the first case of heterothermy in a free-living ruminant in a desert environment: T_b varied by 4.1±1.7°C day^–1 during summer (June to September) and by 1.5±0.6°C day ^–1 during winter (November to March). Over both seasons, mean T_b was 38.4±1.3°C. During the day in both summer and winter, T_b increased continually, suggesting that oryx store heat instead of dissipating it by evaporation, whereas at night T_b decreased. The minimum T_b was lower in summer (36.5±1.16°C) than in winter (37.5±0.51°C) despite the fact that the temperature gradient between T_b and air temperature (T_a) was larger and solar radiation was lower in winter. Throughout the year, daily variation in T_b appeared to reflect thermal load (T_a,max–T_a,min) rather than an endogenous rhythm. Behavioural thermoregulation was used by oryx to cope with thermal stress during summer: animals lay down in shade in the morning shortly before T_a exceeded T_b and remained there until evening when T_b–T_a became positive. The use of heterothermy by oryx resulted in storage ofandwould672.4 kJ day^–1 animal^–1258.6 kJ day^–1 animal^–1in winter, if heat storage is based on calculations involving mean T_b. Todissipaterequirethisheatin summerby evaporation0.28 litres H₂O day^–1 animal^–1and0.11 litres H₂O day^–1 animal^–1 in summer and winter, respectively. Without heat storage in summer, we estimated that oryx would have to increase their water intake by 19%, a requirement that would be difficult to meet in their desert environment. If heat storage was calculated based on the daily change in T_b rather than on heat storage above mean T_b then we estimated that oryx saved 0.538 litres H₂O day^–1 animal^–1 during summer.

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