Heterothermy of free-living Arabian sand gazelles (Gazella subgutturosa marica) in a desert environment
Journal of Experimental Biology
When exposed to high thermal loads, many endotherms maintain their body temperature (T b ) within narrow limits (less than ±2°C), either behaviourally, such as seeking shade, or by panting or sweating, these requiring evaporative water loss (Jessen, 2001) . Species indigenous to desert environments, without access to drinking water, can ill afford profligate use of evaporative water for cooling, and therefore must minimize reliance on this mechanism for maintenance of T b (Wilson, 1989; Degen,
... lson, 1989; Degen, 1997; Schmidt-Nielsen, 1997). One mechanism, thought to be important in reducing evaporative water loss among large desert ungulates, is heterothermy, the storage of body heat during the day, under positive thermal load, and dissipation of this heat at night by non-evaporative means, thereby reducing evaporative water loss in maintenance of normothermic T b (International Union of Physiological Sciences Thermal Commission, 1987; Willmer et al., 2000) . The concept of heterothermy was originally espoused by Schmidt-Nielsen et al. (Schmidt-Nielsen et al., 1957) , who measured 24·h rhythms of T b in captive camels (Camelus dromedarius) during summer; T b of camels increased during the day, decreased at night, and variation in T b was amplified when animals were water-deprived. Extending these observations to captive Cape eland (Tragelaphus oryx), Taylor and colleagues demonstrated that, in environmental chambers set to 40°C during the day, but 22°C at night, eland increased To test whether free-living desert ungulates employ heterothermy to reduce water loss, we measured core body temperature (T b ) of six free-living Arabian sand gazelles (Gazella subgutturosa marica), a small desert antelope (12-20·kg) that lives in the deserts of Saudi Arabia, where air temperature (T a ) often exceeds 40°C. We found that the mean daily T b varied by 2.6±0.8°C during summer (June-July) and 1.7±0.3°C during winter (January-February); over both seasons, mean T b was 39.5±0.2°C. During the day, in summer, T b increased by more than 2°C when T a >T b and declined at night when T a <T b , suggesting that gazelles stored heat during day and dissipated it by non evaporative means during night. The minimum T b was lower in summer (38.2±0.5°C) than in winter (38.6±0.3°C) despite the fact that the gradient between T b and T a was larger and solar radiation was lower in winter. Correlation between daily variation of T b and mean, maximal T a s were significant in summer, but not in winter. To dissipate the amount of heat stored by gazelles would require an evaporative water loss of 33.5·ml·H 2 O·day -1 in summer and 23.2·ml·H 2 O·day -1 in winter. We tested whether the amplitude of daily variation in T b was influenced by the level of water provided to six captive sand gazelles maintained under controlled conditions in summer. The daily amplitude of T b was increased by 1.4°C when gazelles were denied drinking water but supplied with pre-formed water in food, and by 1.1°C when they were denied both water and food. Gazelles denied only drinking water increased the amplitude of variation in T b , whereas when denied both food and water, they seemed to undergo a dehydration-hyperthermia, with increased mean and maximal T b values but no decrease of minimal T b . Free-ranging and captive gazelles surviving on preformed water in natural food used heterothermy during summer with no elevation of plasma osmolality, indicating that they were not in a state of dehydration. Our data on variation in T b of gazelles provide an example of a small desert ungulate employing heterothermy to reduce evaporative water loss that would otherwise be required to maintain normothermic T b .