ANIMALS— PHYSICAL ENVIRONMENT 89 



and horses cannot swallow ; when water is available 

 their consumption is low, about seven gallons daily 

 compared with the horse's thirty. Other large 

 creatures which are able to dispense with drinking, 

 but which drink when they have opportimity, are 

 the ostrich, the giraffe, the eland, and probably 

 many other antelopes from the parts of tropical 

 Africa which are rather semi-arid grass-land than 

 desert. 



In desert of an extreme tjrpe many of the animals 

 have perforce to dispense entirely with drinking 

 water, and it appears that a certain number of 

 species never drink at all, and derive their whole 

 water-supply from the desert herbage, and possibly 

 the dew. In this category of total abstainers Abel 

 Chapman mentions the Addax (Fig. 35) and Oryx 

 (Fig. 36) antelopes and the Addra and Sudan 

 gazelles, all of them inhabitants of the Anglo- 

 Egyptian Sudan. He also states that on certain 

 small islands in the Red Sea Gazella arahica exists 

 as a dwarf race, not exceeding one-third of the 

 weight of the normal race, and that these islands 

 are entirely without water. 



It is probable that this abiUty to dispense with 

 water is a character of many of the gazelles (Fig. 37), 

 and that it accounts for the fact that species of the 

 genus are found almost throughout the Great 

 Palsearctic Desert. 



Grinnell and Dixon state that the Round-tailed 

 Ground Squirrel {Citellus tereticaudus) does not drink 

 even when it has the opportunity ; another species, 

 the Desert Antelope Ground Squirrel (Ammospermo- 



