60% 



62 



64 



66 



66 



70 



72 



74 



Figure 2. 



Percentage of water in the body of kangaroo rats which had lived on dry diet for different 

 periods of time. The two ends of each block represent respectively the highest and the lowest 

 value found for the group in question. The mean value is marked with a vertical line across 



the block. 



maintain water balance, which means that the intake of water on the dry diet is suf- 

 ficient to cover their needs for excretion. They can therefore be ejected to be 

 able to live for any length of time on the dry diet. , 



If we want to make an account for the water intake and water output of an ani- 

 mal we have: 



Intake Output 



Drinking water p„o,.^,^.-:«„/ skin 



Evaporation Ij^^gg 



Water in food Water in urine 



Oxidation water 



Water in faeces 



Intake 



Drinking water: For the kangaroo rat the drinking water can be disregarded. No 

 drinking water is available in their natural habitat except on rare occasions after a 

 heavy rain fall. Dew does not occur normally, often the relative humidity only 

 reaches about 40% at night. When kept in captivity on the experimental diet of dry 

 barley, no drinking water was given. 



Wafer in food: The content of free water in the food, also called preformed water 

 (to distinguish it from the water formed by oxidation) is quite low when the diet 

 consists of dry barley. Grain is hygroscopic and its water content will therefore 

 vary with the relative humidity of the surrounding air. Determinations of the per- 

 centage of water in barley at different humidities gave the results in Table 1. 



By storing seeds and other plant material in the more humid burrow, the ani- 

 mal can increase its water content somewhat. The kangaroo rats have large 

 storages of food in their burrows. 



Oxidation water: In all animals water is formed by the oxidative metabolism. The 

 amount of water formed when a certain amount of any foodstuff is combusted can 

 easily be computed when we know the composition of the foodstuff. Table 2 shows 



175 



