ANIMALS THAT LIVE WITHOUT WATER I55 



wells. The wild ass is believed to subsist without drink- 

 ing water, but the domestic donkey cannot; after an ar- 

 duous day's labor the donkey compensates for water loss 

 by taking a fabulously large drink, consuming as much 

 as 12 per cent of its body weight in the space of fiv3 

 minutes. 



In mammals that are physically active in the hot sun, 

 the control of body temperature by the evaporation of 

 water— whether as sweat, as in man, horses, and cattle; 

 by panting, as in the dog; or both, as in the sheep— 

 may increase the total water requirement far above that 

 involved in urine formation. Sweating presents the ad- 

 ditional comphcation in that it may also cause the loss 

 from the body of considerable quantities of salt. In ani- 

 mals that sweat profusely this loss of salt is such as al- 

 most to keep pace with water loss, and the blood does 

 not become unduly concentrated. Consequently, man, 

 for example, quenches his thirst with a Httle water and 

 only restores the total volume of his body fluid over a 

 period of hours or days as the lost salt is replenished 

 through his dietary intake. If he drinks too much water 

 all at once when in the dehydrated state he may in fact 

 suffer aches and pains from rapid overdilution of the 

 blood. The reason the donkey satisfies its thirst in one 

 big drink (and the dog is rather more like the donkey 

 than like man) is that it does not sweat to so great an 

 extent, and consequently loses httle salt, and the loss of 

 water in respiration leads to concentration of its blood; 

 consequently it drinks enough at one time to bring itself 

 back into water balance. 



Most of the mammals which are knovni authentically to 

 live in the wild state without access to drinking water 

 are of small size. The ground squirrel and pack rat live 

 on succulent vegetation and cacti containing from 60 to 

 90 per cent water, the grasshopper mouse on insects con- 

 taining 60 to 85 per cent water. The common wild hares 

 of the United States, Lepus alleni and L. calif ornicus (in- 



