418 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
conductor such as fat, nearly all the heat absorbed will be taken from 
the air, and the body temperature will hardly be affected. If the skin 
is thin, however, with a rich blood supply and no insulating fatty 
layer, heat may readily be withdrawn from the tissues. The con- 
centration of fat into one depot allows the rest of the surface of the 
animal to act as a radiator in this way. 
Fur is also an advantage to desert animals in preventing overheat- 
ing, for it acts as a heat barrier preventing solar radiation from 
reaching the skin, and also in slowing down the conduction of heat 
from the environment to the animal. On the same grounds, clothing 
for man is an advantage in strong sunshine by preventing the absorp- 
tion of radiant energy by the skin, and thus conserving water. But 
the fur must be sufficiently well ventilated to allow evaporation of 
sweat to occur at the base of the hairs so that heat may be drawn from 
the animal itself. If the fur is so thick that sweat travels through 
the layer in liquid form, evaporation occurs at the outer surface, heat 
is withdrawn from the air and the animal does not benefit. 
We may compare the camel’s performance with that of a man and 
a dog. Both these animals begin to evaporate water as soon as the 
body temperature rises above normal; man from the skin, and the 
dog from the respiratory surfaces. In this respect they are at a dis- 
advantage as compared with the camel, which as we have seen allows 
its body temperature to rise. 
In man, sweating increases with increasing heat load, and may 
reach 1.5 liters an hour. It continues at this high rate in spite of 
progressive desiccation. Renal loss decreases until the maximum 
urine concentration is reached, but this still involves about 0.5 liter 
of water a day. Maximum urine concentration in man is not very 
high, but even a doubling of the permissible concentration would not 
be very effective in desert conditions, for up to 50 times as much water 
is lost by sweating, and a saving of 250 ml. would be insignificant. 
When supplied with water, however, man’s capacity for physical 
work in deserts is better than that of the dog. This was made evi- 
dent in an experiment (Dill, Bock, and Edwards, 1933) where a man 
and a dog walked a course in a desert when the air temperature was 
about 40° C. (104° F.). The man made five rounds, covering 20 
miles, while the dog was completely exhausted after 16 miles. The 
man’s skin temperature remained at about 34° C., but that of the 
dog rose, as a result of isolation, to about 45° C. After each round 
of 4 miles, both were offered water. The dog drank sufficient to 
restore its original body weight, but the man drank porportionally 
less and lost 3 kg. out of 75 kg. in 7 hours, which is equivalent to 4.2 
percent of his body weight. There is an interesting reason for this. 
The sweat of man, although less concentrated than blood serum, con- 
