loss through faeces, 0.76 g is again superimposed on the two curves giving the 

 minimum total water output when lOOkcal of barley are metabolized. 



For water intake we have the oxidation water which, of course, is independent 

 of the environmental humidity. The amount of preformed water in the barley increa- 

 ses with increasing humidity. The top curve gives the total water intake when 100 

 kcal of barley are metabolized. 



From the diagram it can be seen that the water intake exceeds the minimum 

 water output at all humidities above 2.2 mg HjO per litre air or 10% relative humidity 

 at 25°C. Below this value the water output exceeds the water intake, and the ani- 

 mals are in negative water balance. 



Fig. 5 shows a similar diagram for white rats. It is seen that white rats have a 

 considerably higher water output and cannot be in positive water balance at any 

 humidities when they do not get drinking water with the barley. 



The results in the diagrams were obtained by calculation. It was desirable to 

 check them by actual determinations of the animals' response to changes in the en- 

 vironmental humidity. A group of kangaroo rats was kept at different controlled 

 humidities at 25°C on a barley diet for periods around 10 days. The results showed 

 that the animals can maintain or. gain body weight at humidities above 10% relative 

 humidity. At 10% relative humidity and lower the animals lose body weight. )Xhite 

 rats were unable to maintain body weight even at 90% relative humidity when not 

 given additional water. 



In its natural habitat with its relatively humid burrow the kangaroo rat will 

 have a certain margin of safety. The absolute humidities which are measured in 

 their burrows vary between 7 and 14 mg of water per litre air. The humidity outside 

 is considerably lower also at night. By its remarkable ability to conserve water 

 the kangaroo rat shows a high degree of adaptation to its arid habitat. 



181 



