156 FROM FISH TO PHILOSOPHER 



correctly called 'jack rabbits'), and the prairie dog also 

 live on succulent plants; they can be maintained in cap- 

 tivity on a mixture of greens and dry foods, but not on 

 dry food alone. All of them will drink water when availa- 

 ble, especially if they are kept on a dry diet. 



At the extreme, the mammals most successful in hv- 

 ing without free water are the kangaroo rats: Dipus, 

 Jacultis, GerbilluSy Meriones, Dipodillus of the Great 

 Palearctic deserts, Pedetes of South Africa, Dipodomys 

 and Perognathus of the American deserts, and Notomys 

 and Ascopharynx of Australia. Though belonging to dif- 

 ferent famihes, these rodents are all small, they all have 

 elongated hind legs and a long tail, and they jump in a 

 kangaroolike manner, from which habit the group de- 

 rives its popular name. Several of them have cheek or 

 throat pouches for carrying seeds, in the manner of the 

 squirrel, and they are generally as skillful as the squirrel 

 in using the front paws as hands. 



Several of the American kangaroo rats, Dipodomys 

 merriami and D. spectahilis, and Bailey's pocket mouse, 

 Perognathus baileyi, have in recent years been exten- 

 sively studied by Knut and Bodil Schmidt-Nielsen and 

 are the only desert-living mammals on which detailed 

 physiological information is available. These small ani- 

 mals normally live on air-dried seeds and can apparently 

 grow and reproduce and nurse their young on a diet 

 of dry grain or oatmeal containing only 5 to 10 per cent 

 of free water. Actually, they have to be taught to drink 

 water, and v/hile Dipodomys will eat succulent food, 

 P. baileyi disdains green things and will not even eat 

 fresh watermelon. 



Dipodomys has no mechanism for storing water in the 

 body when it is living on moist food, nor does it lose 

 significant quantities of water during a long period on 

 dry food. It can, however, be forcibly dehydrated by 

 feeding it a high-protein diet of dried soybeans (40 per 

 cent protein), which causes increased loss of water in 

 the urine in consequence of the large quantity of urea 



