COMMUNITIES IX DRY, OPEN LANDS 



447 



the so-called compass nests in northern Australia, which are long, 

 narrow, and high, and without exception turn their narrow axis north 

 and south, so that they offer a narrow surface to the strongest rays of 

 the sun (Fig. 119), but are equally exposed to its rays on both sides. 

 Burrowing is common among the reptiles of open country. Tor- 

 toises like Testudo horsfleldi and the monitor lizard {Varanus niloticus) 

 are able to burrow even into the driest loam; the skink (Scincus) fairly 

 swims through loose sand; and short-bodied lizards in various parts 

 of the world burrow into sand by tipping the body from side to side. 



Fig. 119. — ''Compass nest" of an Australian termite, from Port Darwin, North 



Australia. After Saville Kent. 



Many snakes burrow in the soil and have modified snouts or other 

 characteristics in adaptation to this habit. 



Rodents are the most numerous burrowing mammals; they are 

 found in all parts of the earth, and are often structurally similar be- 

 cause of similar modes of living despite remoteness of relationship. 

 They dwell side by side in suitable places in the open country in 

 such numbers that the ground is undermined over wide stretches. 

 Saddle horses are contantly endangered by the caving in of the bur- 

 rows of Pedetes and Xerus in the South African steppes, 8 of Ctenomys 9 

 in the Patagonian plains, of the whistling hares 10 in the Mongolian 

 steppes, or of the prairie dog in the western United States. Numerous 

 mammals of other orders must be added to the list of burrowers in 

 open lands. The subsoil thus brought to the surface supports a type of 



