HUDSON 



ficult. However, it is possible to compare the abilities of C. leu - 

 curus and other desert rodents to minimize excretory water loss. 

 Such a comparison serves as a basis for acquiring insight into 

 their relative dependence on water ingestion. 



The capacity of some of the heteromyids to keep urinary water 

 loss at a minimum by the production of a very concentrated urine is 

 well known. However, there is little information on other desert 

 species. Direct comparison of renal concentrating capacity among 

 species which may differ slightly in kidney performance is com- 

 plicated by the variability of kidney function; this is in part related 

 to variations in ambient temperature, diet, and fluid intake. For 

 example, animals given water ad libitum show a correlation between 

 the urine concentration and ambient temperatures (Fig. 10). Further- 

 more, because of the possibility of active transport of urea in the 

 renal tubules (B. Schmidt- Nielsen, 1960), a high protein diet may 

 increase solute excretion without causing an appreciable increase 

 in excretory water loss. Single measurements of urine concentration 

 in animals deprived of water tells little of the minimum daily water 

 loss required for the discharge of excretory wastes. 



One useful technique for comparing different species is to 

 measure the concentration of urine produced over a 24 hour period 

 (with comparable diets) when a species is drinking only enough water 

 to maintain body weight. Data on average urine concentration per 

 24 hours while drinking a quantity of water minimal for weight 

 maintenance are presented for C. tereticaudus (Fig. 11). 



In order to compare the renal concentrating capacity of C. 

 tereticaudus with other species, it is necessary to assume that the 

 serum has a solute concentration of approximately 350 milliosmols 

 and then to divide the urine concentration by this figure. On the 

 basis of this assumption, the daily urine concentration of C. teret- 

 icaudus averages eight times the serum concentration. The average 

 ratio of urine and serum concentrations in the kangaroo rat ( D. 

 merriami ) as estimated from the data of Schmidt-Nielsen et al. 

 (1948a,' 1948b) is 10.3 when the animals are on a normal diet and 12.1 

 when animals are eating soybeans. The antelope ground squirrel 

 has a urine-serum ratio of 9.7 when deprived of water (Fig. 12). 

 Although values from all of the species are difficult to compare, it 



438 



