Heterothermy and the water economy of free-living Arabian oryx (Oryx leucoryx)

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Abstract
SUMMARY To test the idea that large, free-living, desert ungulates use heterothermy to reduce water loss, we measured core body temperature (Tb) 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: Tb 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 Tb was 38.4±1.3°C. During the day in both summer and winter, Tb increased continually, suggesting that oryx store heat instead of dissipating it by evaporation, whereas at night Tb decreased. The minimum Tb 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 Tb and air temperature (Ta) was larger and solar radiation was lower in winter. Throughout the year, daily variation in Tb appeared to reflect thermal load (Ta,max–Ta,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 Ta exceeded Tb and remained there until evening when Tb–Ta 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 Tb. 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 Tb rather than on heat storage above mean Tb then we estimated that oryx saved 0.538 litres H2O day–1 animal–1 during summer.

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