ANIMALS AND ARID CONDITIONS 375 
body temperature has a double advantage. In hot environments 
the camel, instead of evaporating water, permits the body tem- 
perature to increase to a maximum of about 40.6°. This increase 
can be regarded as a storage of heat in the body, heat which can 
be dissipated in the cooler night without expense of water. But 
there is one further important advantage to the high body tem- 
perature. The difference in temperature between the hot environ- 
ment and the body will be smaller, and since the movement of 
heat is proportional to the temperature difference, less heat 
reaches the body and less water is required to prevent a further 
increase in body temperature. 
This reaction, which is a well-regulated physiological mecha- 
nism, effects a very considerable economy with water in the two 
ways Just mentioned. 
Length of Time Without Drinking 
The time an animal can tolerate lack of water in the desert 
depends on the rate of water loss, and the limit to which actual 
desiccation of the body can be tolerated. We have just seen some 
of the physiological mechanisms which result in an exceptionally 
low rate of water loss in the camel. They are so efficient that the 
rate of water loss in this animal in the summer is less than one- 
third that of the donkey, which is also well adapted to the desert 
climate. 
The camel can also withstand an exceptional degree of dehy- 
dration of the body. In one case a camel had lost more than 40% 
of its body water before it was allowed to drink. This should be 
compared with the situation in man and other mammals which 
die from explosive heat rise when 15-20% of the body water have 
been lost in hot surroundings. However, the donkey can tolerate 
as much depletion of the body water as the camel, showing that 
this animal too is exceptionally well adapted to desert life. 
The camel, since it has a low rate of water loss and tolerates 
considerable depletion of the body water, can go for a long time 
without drinking. How long he can go without water depends 
not only on external conditions of air temperature, wind, solar 
radiation, etc., but also on how much he works, how far he has 
