WATER CONSERVATION IN SMALL DESERT RODENTS 



Dr Bodil Schmidt -Nielsen 

 (Cincinnati, U.S.A.) 



Animals inhabiting deserts have in the course of evolution acquired morpho- 

 logical and physiological characteristics which enable them to live £ind thrive in an 

 environment that is hostile and uninhabitable to other closely related forms. 



Among rodents, specially adapted desert forms are found in all the major deserts 

 around the world. It is interesting that these rodents, although they belong to dif- 

 ferent families are similarly adapted to their environment. 



In the north American deserts the kangaroo rats and pocket mice (Dipodomys 

 and Perognathus) of the family Heteromyidae are found. In the great Palaearctic 

 desert we find Gerbillus, Meriones and Dipodillus, belonging to the family Muridae; 

 and Jaculus, Dipus and Alactaga of the family Dipodidae. In South Africa, in the 

 Kalahari desert, the rodent Pedetes, family Pedetidae, is found; and in the Austra- 

 lian deserts the family Muridae is represented by Notomys. All these rodents have 

 several morphological features in common (Fig. 1) thus they are all adapted to a 

 bipedal saltatorial life with elongated hindlegs and reduced number of toes. Several 

 of them have cheek pockets (Heteromyidae)or gular pouch {Notomys fuscus and 

 cervinus) and they all have greatly inflated bullae auditivae. They are nocturnal 

 and stay in their underground burrows during the daytime. 



Also with respect to their physiological adaptation to the environment do we 

 find striking similarities between these animals. 



Personally we have worked primarily with kangaroo rats (Dipodomys) but we 

 have also had the opportunity to work with jerboas from Arabia and we found that 

 their water problem has been solved in the same way as the kangaroo rats' has. 



It seems likely that the same would be true of many other desert rodents. 



Can the desert rodents live without water? 



When we started to investigate the water metabolism of the kangaroo rats the 

 first question to be answered was whether these animals can live entirely without 

 drinking water. Previous reports in the literature indicated that this was the case 

 but more accurate investigation was desirable to solve this problem. We kept kan- 

 garoo rats on diets of dry grain (rolled barley), without water. They maintained 

 their body weight, and some even gained body weight over a period of 2 months. 



The next question was: Can the animals during a period of water deprivation 

 store their metabolic waste products to avoid spending water for excretion? This 

 is practiced by the lungfish which stores urea in its body when estivating in the 

 dry mud. The stored urea can amount to 2-4% of the body weight. Urea and elec- 

 trolyte concentrations were determined in the plasma of (1) kangaroo rats that were 

 freshly trapped, (2) kangaroo rats that had been on a moist xjiet (barley and water- 

 melon) and (3) kangaroo rats that had been fed on dry barley only from 2-8 weeks. 

 The same average urea and electrolyte concentration of the plasma was found in all 



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