COMMUNITIES IN DRY, OPEN LANDS 445 



toleration of high temperatures. Birds survive in dry areas because 

 their powers of flight bridge the stretches between watering places 

 and because in the main they are sparing in their use of water. 



Mammals are principally represented by forms relatively inde- 

 pendent of water either because of the slight development of the 

 cutaneous glands (rodents), concentrated urine (antelopes), dry faeces, 

 or all these. Many rodents, a large number of antelopes, the aardvark, 

 and many armadillos (e.g., Dasypus minutus), may survive for months 

 without drinking; for these, the limited water which they take up 

 with their food, with bulbs and tubers, with melons and succulent 

 plants, together with the water produced in the processes of metabo- 

 lism, is sufficient to satisfy their fluid requirements. Camels are able 

 to get along for five days, even ten to twelve days if necessary, with- 

 out water. Elephants, except for herds with young calves, need to 

 come to water only every two or three days. Many ungulates, like 

 the zebra and white-bearded gnu {Connochaetus albojubatus) , do not 

 hesitate even to drink salt water, such as is found in many steppes; 

 indeed they thrive on it. 5 On the other hand, rhinoceros, water buck 

 (Cobus) and reedbuck {Cervicapra) , monkeys, and even rabbits are 

 never too far from water to quench their thirst daily. Many mam- 

 mals of the steppes have a keen sense of smell for moisture. Distant 

 rains in the South African veld cause a migration of wild animals, and 

 elephants are attracted from a distance of 100 km. by the first down- 

 pour of the wet season. 6 



Temperature and winds. — A universal characteristic of open 

 lands is the fluctuation of temperature, particularly from day to night, 

 but often also between summer and winter. There is no protecting 

 leafy roof to ward off the rays of the sun by day and to prevent radia- 

 tion by night, which is further favored by a cloudless sky. In the 

 Cameroon savanna, a mid-day heat of nearly 40°C. in the shade is 

 followed by a night temperature of 3° ; indeed, in the Sahara a night 

 frost of —5° following even higher day temperatures may occur, and 

 in southwestern Africa, fluctuations of 40° within twenty-four hours 

 are not unknown. In the Arizona deserts, sudden wind changes may 

 bring temperature rises of 36° within eight hours. In high Tibet, mid- 

 day heat reaches almost 40° in summer, and the winter temperature 

 may descend to —37° and lower. There is, therefore, no place for 

 surface-dwelling stenothermic animals in such regions, and even the 

 eurythermic forms that are found here need protection against ex- 

 tremes of both heat and cold. This applies even to the homoiothermic 

 birds and mammals, and especially in completely open plains, where 

 there is no shelter from the direct rays of the sun. 



