below 30°C. The animal then by its nocturnal habits avoids the extreme heat of 

 the day and can therefore avoid spending water for heat regulation. 



Urine: Water can be saved by increasing the concentration of the solids in the 

 urine. This is done to a high degree by the kangaroo rats. Table 4 shows the 

 maximum urine concentrations in man, white rat and kangaroo rats. The kangaroo 

 rat can excrete a much more concentrated urine than can man and the white rat. 



TABLE 4 

 MAXIMUM CONCENTRATIONS OF ELECTROLYTES AND UREA IN URINE 



Electrolytes Urea 



Man 0.37N (2.2%) l.OM (6%) 



Norway rat 0.60N (3.5%) 2.5M(15%) 



Kangaroo rat 1.2 N (7%) 3.8M(23%) 



With respect to electrolytes a kangaroo rat can excrete a urine that is twice as 

 concentrated as sea water. This brought up the old question whether sea water can 

 be utilized as drinking water by mammals. To test this kangaroo rats were fed on a 

 diet of soy beans. On this high protein diet, the animals cannot maintain water 

 balance without additional water. They were offered sea water to drink. A control 

 group was given soy beans and tap water. From Fig. 3 it is seen that both groups 

 of animals lost weight initially until they learned to drink, then they increased in 

 body weight until they reached a steady state. The animals on sea water were 

 doing just as well as the animals on fresh water. Sea water can then be utilized as 

 drinking water by the kangaroo rat. 



Faeces: The faeces excreted by the kangaroo rat are very dry compared with the 

 faeces of the white rat. Determinations of the moisture content in the faeces of 

 kangaroo rats and white rats and determinations of the amount of faeces eliminated 

 when a certain amount of food was metabolized showed that the kangaroo rat looses 

 only 0.76g of water in the faeces when lOOkcal of barley is metabolized while the 

 white rat looses 3.4g of water when the same amount of food is metabolized. 



Complete account for intake and output of water 



With the information obtained above it is now possible to calculate at what 

 humidities in the surrounding air the kangaroo rat is able to maintain water balance. 

 Fig. 4 shows the result of the calculation. The calculation is based on the intake 

 and metabolism of 100 kcal of barley, corresponding to 25 g of dry pearled barley. 

 The ordinate gives the water intake and the minimum water output in grams per 100 

 kcal of pearled barley metabolized. The abscissa gives the humidity in the environ- 

 mental air. 



178 



