Keeping the head cool

Recently Taylor has shown that gazelles and antelopes may reach body temperatures over 46°C, apparently without harm. The oryx (Oryx beisa), which weighs about 100 kg, is extremely tolerant to hot dry conditions and can stand in the hot sun all day. When kept on a regimen of dry fodder without drinking, its oxygen consumption decreases substantially, and this reduces the internal heat load. At very high air temperature, however, the oryx will evaporate water by panting, but since it permits its body temperature to increase to above 45°C, the amount of water used is drastically reduced. At times Taylor's oryx tolerated a rectal temperature of 46.5°C for as long as 6 hours without observable ill effects.

How can a mammal withstand such high internal temperatures? The brain and central nervous system are probably the parts most sensitive to heat, and a tolerance of 46°C or more would be unique among higher animals (Taylor, 1970). The answer is simply that the brain does not reach the same high temperature as the body core. In a gazelle in which the body temperature had been driven up by exercise, the brain temperature proved to be as much as 2.9°C lower than in the central arteries (Taylor, 1969). The blood running to the brain is cooled owing to a peculiar arrangement of the blood vessels in the head. The external carotid artery, which supplies most of the blood to the brain, passes through the cavernous sinus, here it divides into hundreds of small arteries which again join into one vessel before entering the brain.