128 



PHYSIOLOGICAL REGULATIONS 



('31), rats attempted to reach an accustomed source of water sup- 

 ply much more frequently after one day's privation of water than 

 ordinarily. With further privation fewer attempts were made; 

 possibly this is related to extinction of the unrewarded response. 



A radically different criterion of water deficit is required in the 

 rat from that of initial body weight used in larger mammals and 

 frog. Whereas in dog and man the food intake is suitably main- 

 tained even when water is denied, in rat the food intake is dimin- 

 ished. This diminution itself reduces the body weight of the rat, 

 and represents an appreciable error only in mammals so small that 

 they metabolize rapidly per unit of weight. Water balance, then, 

 is at less than the initial weight. Of the various methods of evalu- 





2 - 



o 



lU 

 O 



on 







urinary 

 ^evaporative 



-2 +2 



Total Water Load 



Fig. 77. Initial rate of water exchange (% of BoAour) in relation to administered 

 water load (% of Bo) (Equilibration diagram for water). Eat in first 1.0-hour of re- 

 covery. Numerals indicate the number of tests. At the dash lines the hour 's exchanges 

 would equal the water loads. New data. 



ating the new balance, the one chosen is to find the weights of con- 

 trol rats supplied with water ad libitum but limited to those 

 amounts of food consumed by the individuals to which water was 

 denied. The actual loads of water being thus established, ex- 

 changes are compared in the initial hour of recovery (fig. 77). 

 Total gain is the water drunk plus water formed by oxidation; 

 water lost is the total gain minus the measured gain of body weight 

 in the one hour of recovery ; part of the loss is represented by urine 

 collected. Clearly the rat takes slightly more than enough water 

 to restore water balance, just as the dog and rabbit do. The chief 

 differences are that the rat requires nearly an hour to ingest this 

 water instead of the few minutes used by those species, and that 

 initial body weight is an insufficient measure of zero load of water. 



