tract. If we calculate the amount of water evaporated from the lungs of man in dry 

 air we arrive at 0.84 ml of water per ml of oxygen utilized. In kangaroo rats and 

 some other small rodents we measured the total evaporation (body surface and res- 

 piratory tract combined) in dry air, simultaneously with the oxygen uptake. The 

 results are listed in Table 3. The total evaporation in the kangaroo rats per ml oxy- 

 gen taken up is very low compared with the evaporation from the lungs alone in man. 

 In the white rat the total evaporation is approximately the same as from the lungs of 

 man. 



The explanation for the low evaporation in the kangaroo rats is that the ex- 

 pired air is saturated with moisture at the temperature of the nose which is about 

 10 C lower than that of the body. The white rats also have a low nose temperature 

 and low evaporation from the lungs. The higher total evaporation from the white 

 rats can probably be accounted for by a higher transpiration from other parts of the 

 body. The evaporation from the skin is negligible for the kangaroo rats. The eva- 

 poration from the lungs decreases with increasing amount of water vapour in the 

 inspired air. 



Ue measured the relative humidity and temperature in the burrows of the kan- 

 garoo rats in order to determine how much water the animals would save by staying 

 in their underground burrow. The measurements were done by tying small micro- 

 climate recorders to the tails of kangaroo rats and releasing the animals in front of 

 their own burrows. The animal would carry the instrument to its nest chamber. The 

 instrument could then be dug out after the humidity and temperature had been recor- 

 ded for 10-12 hours. 



The measurements showed that the absolute humidity in the burrows is about 3 

 to 4 times as high as the simultaneous humidity outside the burrow, which means a 

 considerable reduction in the amount of water evaporated from the respiratory tract. 

 The temperature in the burrow is much more constant than outside and always stays 



177 



