ANIMAL SURVIVAL IN HOT DESERTS—EDNEY 419 
tains salt, and he drank only enough water to restore the normal 
blood salt concentration. The dog transpires from the lungs and thus 
loses no salt, so that it was able, by drinking, to restore all the water 
lost without lowering its blood-salt concentration. Dogs have the 
advantage of men in this respect. 
Total evaporation from the dog was 2.6 percent of body weight 
per hour, that from the man was 1.74 percent; but if measured in 
terms of surface area, evaporation from the dog was lower: namely, 
0.65 1./hr./m.? against 0.72 l.hr/m.? from the man. Since the body 
temperature of both remained constant, the difference in evaporation 
per unit area should reflect a difference in heat load Heat is gained 
by metabolism, radiation, and conduction from the air. Metabolism 
is proportional to surface area in both animals and therefore adds an 
equal load to each; the radiation load, however, is lower in the dog be- 
cause its skin temperature is higher. Thus when the surface tempera- 
ture of the environment is 65° C. and a dog’s skin temperature is 45° C., 
the gradient is 20° C. In man, with a skin temperature of 35° C., the 
gradient is steeper. 
Again, heat flow by conduction to the air is outward in the dog 
(45° C. skin temperature and 40° C. air temperature), whereas in 
man heat flow is in the reverse direction. In this situation the dog, 
by not sweating from the skin and therefore having a higher skin 
temperature, had an advantage over man as far as water economy 
is concerned. However, long-term advantages in water conservation 
do not necessarily correspond with conditions of immediate comfort. 
A skin temperature of 45° C. in man would not be tolerable for long. 
Strictly comparable figures for camels are not available, but the 
overriding fact is that a camel can tolerate a loss of water up to 
nearly 25 percent of its body weight while a man succumbs after 
losing 10 to 12 percent. 
Summarizing the above, we see that camels, men, dogs, and kanga- 
roo rats, all of them mammals, conform to expectations. The small 
ones avoid extreme conditions, and the large ones resist them by 
sweating. In addition, special adaptations, by modification of the 
general mammalian physiological plan, are apparent in typical desert 
inhabitants. 
ARTHROPODS 
We may now examine the situation in desert arthropods. Here 
we have to do with a rather different physiological pattern. All 
these animals are so small that active regulation of body tempera- 
ture by evaporation of water is impossible, at any rate for more than 
very short periods, and water conservation is all important. They 
are all poikilotherms, and their temperatures fluctuate with that of 
the environment, so that toleration of a wide range of body tempera- 
