502 LAND ANIMALS 



ful yak, for example, which survives on pasture in which cattle would 

 starve, seems to prefer hard and dry grass, and eats lichens and moss. 

 Many mammals of high altitudes are more sensitive to heat in summer 

 than to cold in winter. Food scarcity and severe weather can be avoided 

 by migration, and alpine mammals make regular seasonal migrations 

 upward and downward. The marmot moves downward to hibernate at 

 the timber line. Areas with much snow and unusually snowy seasons 

 increase these downward migrations. 54 The increase of size in the 

 colder zones, in accordance with the Bergmann Rule, is a familiar 

 phenomenon in the alpine zone. Thus in the Alps the wood mouse 

 (Mus sylvaticus) and the meadow mice* are larger in the alpine zone 

 than in the valleys. 55 The short-eared and compact Microtinae range 

 to greater heights than the Muridae. The alpine shrewsf are among 

 the largest forms of their groups. 



The reduced competition in the alpine zone has led a considerable 

 number of mammals to enter this treeless zone; many of them are 

 relatively primitive. Most of the truly alpine mammals are herbivores. 

 The predaceous mammals are mostly euryzonal forms, which range 

 into the alpine zone only when there is a sufficient food supply to 

 tempt them. The snow leopard seems to be exceptional in this respect, 

 being somewhat more closely confined to the high levels in central 

 Asia. The herbivores include rodents, ruminants, and hyraces. Some 

 of the more northern mustelids in North America, such as the wolverine, 

 range southward in the Rocky Mountains, where they are confined to 

 the alpine zone. 



The smaller alpine rodents include numerous euryzonal forms, but 

 some, such as Microtus nivalis of the Alps, are stenozonal and range 

 to 3500 m., the highest altitude reached by mammals in Europe. The 

 larger forms, like the marmots, range to 3000 m. in the Alps and central 

 Asia. The chinchillas and their allies (Lagidium) in the Andes are 

 abundant at 3000 m., and range to 5000 m. 



The ruminants include a wide variety of forms, but principally 

 sheep, goats, and (in the Old World) antelopes. The yak of central 

 Asia is the only bovid of high altitudes, and the musk deer of the same 

 region represents an isolated group among the deer. Wild sheep and 

 goats seem to be almost entirely mountain animals, sure-footed and 

 capable of extraordinary leaps. The antelopes are fewer, mostly stock - 

 ily built and short-necked forms, which are usually associated as the 

 subfamily Rupicaprinae. The chamois (Rupicapra) of the Pyrenees, 



* Evotomys glareolus nageri and Microtus arvalis. 

 t Sorex alpinus and Crossopus fodiens. 



