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Inside JEB |
HOT ORYX OPTS FOR HETEROTHERMY
kathryn{at}biologists.com
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By desert standards, 2001 was a wet year in Saudi Arabia, with almost 14 cm of rain falling. But even in the wettest years, water is scarce, and mostly locked up in the few hardy plants that tolerate the harsh conditions. Which is why the Arabian oryx never drinks! It extracts every drop of water that it needs from its diet of grass and shrubs. But as the daily temperatures rise, how can a large mammal protect itself from the threat of dehydration? Under some conditions, camels and other ungulates seem to prevent themselves from dehydrating by simply absorbing heat, rather than losing precious fluid by sweating. But no one had ever seen a case of heterothermy outside the lab in a large endotherm. Knowing that the Arabian oryx had adapted to survive one of the planet's driest environments, Stephane Ostrowski and his colleagues took to the desert to track the oryxes' body temperature as the animals roamed the desert, gathering the first clear evidence that a large mammal resorts to heterothermy in the wild (p. 1471).
The Arabian oryx is one of those rare animals that returned from the brink of extinction when it was successfully reintroduced into the Arabian Desert in the 1980s. By 1996, when Ostrowski joined the team of conservators at the National Wildlife Center, the population had reached almost 500 in the wild. As the oryxes' survival was relatively assured, Ostrowski decided that it was time to learn more about this remarkable animal's physiology.
Working with Joseph Williams and Khairi Ismael, he fitted temperature sensitive radio transmitters and tracking devices to six young adults. But tracking the oryx over more than 2000 km2 of desert was far from straight forward! With the help of a team of local rangers, Ostrowski and Ismael followed the animals' progress over a two year period, collecting over 800 hours of temperature data and recording the animals' behaviour throughout the day.
Not surprisingly, during the summer the animals spent a large part of the day sheltering from the heat, and sure enough, as each day wore on, the animal's body temperature gradually rose from 36°C in the morning to over 40°C at sunset! But could its ability to store heat really protect the oryx from dehydration? Ostrowski calculated how much energy the animal stored during the day, and then calculated how much water the animal would have lost if it had stayed cool by either sweating or panting; the oryx had saved 0.5 l, almost one third of the animal's daily water requirements!
The team then compared the animals' temperature fluctuation in the winter, and were astonished when they realised that the oryxes' minimal body temperature remained higher than in the summer, despite the cooler weather. But Ostrowski can explain this apparent paradox. Even an oryx begins losing water when its temperature reaches 41°C, so in summer it could be in serious danger of overheating, unless it could drop it's body temperature low enough at the start of the day, to store the extra summer heat. So in winter, when the heat is less threatening the animal never needs to go as low as it does in the summer.
References
Ostrowski, S., Williams, J. B. and Ismael, K.
(2003). Heterothermy and the water economy of free-living Arabian
oryx (Oryx leucoryx). J. Exp. Biol.
206,1471
-1477.
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