158 FROM FISH TO PHILOSOPHER 



rhea, but diarrhea is not reported, which is the more sur- 

 prising since the animal drank quantities of sea water 

 ranging from 3 to 15 per cent of the body weight per 

 day. 



If an aglomerular mammal were possible one would 

 expect to find it here, but the glomeruli of the kangaroo 

 rat are typically mammalian, and the rate of glomerular 

 filtration is of the same magnitude per unit of body 

 weight as in the laboratory white rat, an animal not par- 

 ticularly specialized for arid Hfe. Calculated on the basis 

 of oxygen consumption, the filtration rate in the kanga- 

 roo rat ranges from 0.27 to 0.48 cc. per cc. of oxygen, 

 figures to be compared with the range of 0.35 to 0.75 

 cc. in man. Clearly the filtration process is not severely 

 curtailed, despite the fact that the average normal urine 

 output is only one-tenth (0.0004 ^^■) ^^ ^^^ ^^ ^^^ 

 (0.004 cc. per cc. of oxygen consumed). 



Perhaps the rodents are geologically too young to per- 

 mit degeneration of the glomeruli, but an alternative ex- 

 planation is more attractive: when, in the evolution of 

 the mammalian kidney, the renal-portal system was 

 abandoned, the kidney got into a blind alley of its own 

 —the filtration-reabsorption system is now so firmly es- 

 tabhshed that there is no easy way to overhaul it and 

 to convert it to a purely tubular kidney, as the marine 

 fishes have done. The crisis of water deprivation can be 

 met only by making a more and more concentrated urine 

 and conserving water to the utmost by every other means 

 —which is just what the kangaroo rat does, breaking all 

 records in these respects. 



Calculated on the basis of oxygen consumption, man 

 loses much more water by evaporation from the lungs 

 alone than Dipodomys loses from the lungs and skin to- 

 gether. One of the adaptations reducing respiratory wa- 

 ter loss in Dipodomys is a long nose, because the longer 

 the nose, the lower the temperature at the end and the 

 less water lost by evaporation into the expired air. Di~ 

 podomys has no longer a nose than the white rat, but the 



