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COMPARATIVE AND EVOLUTIONARY PHYSIOLOGY

Absence of selective brain cooling in unrestrained baboons exposed to heat

¹Physiology, School of Biomedical, Biomolecular and Chemical Science, University of Western Australia, Crawley, Perth, Australia; ²Brain Function Research Group, School of Physiology, University of the Witwatersrand Medical School, Parktown, South Africa; and ³Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming

Submitted 17 November 2006 ; accepted in final form 3 January 2007

To test whether baboons are capable of implementing selective brain cooling, we measured, every 5 min, the temperature in their hypothalamus, carotid arterial bloodstream, and abdominal cavity. The baboons were unrestrained and exposed to 22°C for 7 days and then to a cyclic environment with 15°C at night and 35°C during the day for a further 7 days. During the latter 7 days some of the baboons also were exposed to radiant heat during the day. For three days, during heat exposure, water was withheld. At no time was the hypothalamus cooler than carotid arterial blood, despite brain temperatures above 40°C. With little variation, the hypothalamus was consistently 0.5°C warmer than arterial blood. At high body temperatures, the hypothalamus was sometimes cooler than the abdomen. Abdominal temperature was more variable than arterial blood and tended to exceed arterial blood temperature at higher body temperatures. Hypothalamic temperature cooler than a warm abdomen is not evidence for selective brain cooling. In species that can implement selective brain cooling, the brain is most likely to be cooler than carotid arterial blood when an animal is hyperthermic, during heat exposure, and also dehydrated and undisturbed by human presence. When we exposed baboons to high ambient temperatures while they were water deprived and undisturbed, they never implemented selective brain cooling. We conclude that baboons cannot implement selective brain cooling and can find no convincing evidence that any primate species can do so.

thermoregulation; brain temperature; primates; hominids; dehydration