ANIMAL SURVIVAL IN HOT DESERTS—EDNEY 417 
At the end of such a period of desiccation the camel will drink sufli- 
cient water to restore its body weight (but no more) in a few minutes. 
How is it that a camel can tolerate a much larger proportional loss 
of water than a man? In most mammals subjected to high tempera- 
tures in dry air, desiccation proceeds slowly while the temperature re- 
mains rather constant. But owing to loss of water from the blood, 
the latter becomes gradually more viscous. This puts extra strain on 
the heart, which, at a certain degree of viscosity, cannot circulate the 
blood sufficiently rapidly to carry away metabolic heat to the skin. 
At this point the temperature rises rapidly and death follows sud- 
denly. The phenomenon has been termed “explosive heat death” by 
Adolph. In camels, however, explosive heat death is rather skillfully 
avoided. There is a physiological mechanism, whose nature is so far 
unknown, which ensures that water is lost from the tissues only, 
while the blood volume, and hence its viscosity, remain constant. In 
a camel which lost 50 liters of water, reduction in blood volume was 
found to be less than 1 liter. 
Because of this ability and also because of its relatively small sur- 
face area, the camel can afford to sweat and thus to reduce the body 
temperature. Furthermore it avoids undue stress in this respect by 
allowing its temperature to vary over a greater range than other 
mammals do. In man, the daily fluctuation in temperature is about 
1° C., but the camel’s temperature falls to 34° C. during the cool of the 
night and rises slowly throughout the day to as much as 41° C. To 
raise 450,000 g. through 7° C. needs a lot of heat. Only after this 
temperature is reached does sweating commence. 
The old story that the camel stores water in its hump has by now 
been decently buried, but it has been replaced by another almost 
equally fallacious: that the fat of which the hump is composed is 
essentially a water store itself. As we have seen, it is perfectly valid 
chemistry to say that 100 g. of fat when completely oxidized yield 
107 g. of water. But in order to oxidize the fat and make the water 
available, extra oxygen must be used, and this involves extra loss of 
water through the lungs to an amount which just about cancels any 
gain from the oxidation of fat. 
No, the camel’s hump is a foodstore, just as any other fat deposit 
is, but there is an interesting reason for its taking that form. As 
Schmidt-Nielsen has pointed out, it is curious that in many mammals 
that live in hot climates, fat is not distributed as a subcutaneous 
layer, but is restricted to one large deposit. In camels this is the 
hump, in Indian and Zebu cattle, the same; in fat-tailed sheep it is the 
tail. Now when water evaporates, cooling occurs by the absorption 
of heat from the immediate environment. But if the animal’s sur- 
face is thermally insulated from the rest of the body by a poor heat 
