ANIMAL SURVIVAL IN HOT DESERTS—EDNEY 415 
were confirmed, it would seem that besides the known effect of evapo- 
ration in removing water from the body, there must be another 
mechanism drawing it in. Possibly these two mechanisms are at work 
all the time, the net effect either of uptake or loss, depending upon 
the relative efficiency of the two processes in a given individual under 
given conditions. The nature of the inward force, if it exists, and 
the conditions in which it is apparent, are quite unknown. 
Another source of water, an important and a universal one, is the 
water of metabolism. Al] animals necessarily produce water when 
they oxidize food containing hydrogen, and indeed the amount of 
water produced may be greater than the initial weight of the food. 
Thus 100 g. of fat, completely oxidized, yield 107 g. of water. This 
is the reason for the remarkable observation that certain beetle larvae 
may increase in weight during starvation (Mellanby, 1932). But 
the process is in no way an adaptation to dry conditions. It is com- 
mon to all animals. The adaptation consists in conserving, by other 
means such as we have already discussed, water from metabolism to- 
gether with water from any other source. 
LIFE IN DESERTS 
THE DESERT ENVIRONMENT 
Let us now consider extreme terrestrial conditions, taking a hot 
desert as an example. What is known about the physiology of the 
animals that live there, and what remains to be discovered ? 
In a desert the surface temperature of the ground is very high 
during the daytime. It is often higher than that of the air above it, 
owing to the absorption of radiant energy from the sun. During the 
night, surface temperatures fall very steeply because the sparse cover 
of vegetation permits rapid radiation of heat to the sky, and the air 
is then warmer than the ground. At all times temperatures below the 
ground and in caves are much less extreme than surface temperatures. 
Humidity is usually very low, and this, combined with high tempera- 
ture, leads to rapid evaporation of water from moist surfaces. But 
because of the great fall in temperature at night, the relative humidity 
rises and evaporation is greatly reduced. If the surface temperature 
falls sufficiently low, dew may be formed. In these circumstances, 
during the daytime heat is gained by an animal largely by radiation 
from the sun and from the ground, but also significantly as a result 
of metabolism, especially in large animals. Little heat is gained by 
conduction from the ground. The air temperature may be above or 
below that of the animal’s surface and heat flows by conduction ac- 
cordingly. Heat is lost by evaporation of water and by conduction 
to the air if the latter is below skin temperature. 
536608—60-——28 
