512 LAND ANIMALS 



concealment. White coloration in birds appears in the Antarctic, where 

 land mammals are absent, and in the giant petrel, which appears to 

 have no effective enemies; 18 white coloration is more frequent towards 

 both poles. The importance of concealment in the short and dull days 

 of the polar winter appears to be small. 



Every means of heat conservation for homoiothermal animals in 

 polar regions is of importance, since they depend directly on heat of 

 metabolism during the greater part of the year, and only secondarily 

 on radiant heat from the sun. White animals are accordingly numerous 

 in the polar regions and correspondingly infrequent in the tropics. 

 Ptarmigan, snowy owl, arctic hare, arctic fox, and polar bear are com- 

 pletely white. The Greenland reindeer and the Greenland falcon are 

 very light colored. The number of white wolves increases to the north- 

 ward in North America. The lemmings Dicrostonyx torquatus and 

 hudsonius and the ermine are white in winter. The snow bunting is 

 entirely white beneath and rust brown above, in winter plumage. The 

 Greenland redpoll, though only a summer resident, is the palest of all 

 the redpolls. 



Hibernation is impossible in the arctic winter (cf. p. 409). Deep 

 caves are not available as warmer refuges, since the temperature in 

 the depths is the mean annual temperature, and this is below 0°. The 

 earth is frozen year in and year out below a depth of 30 to 40 cm. 

 In the extreme north, most warm-blooded animals that can do so, 

 migrate. Even the residents such as the snow bunting, falcon, and 

 raven give way before the deep ice and snow, and move to somewhat 

 more southern regions and to the coasts of open seas. The barren- 

 ground caribou of the mainland of America and of Greenland migrates 

 southward in vast herds. The Spitzbergen reindeer does not migrate. 

 Arctic hare and musk oxen maintain their stations, and at most seek 

 out the more favorable spots in their normal range. The ptarmigan 

 digs tunnels in the snow, where it finds both shelter and food. 19 The 

 smaller mammals take refuge from storms in rock crevices or in the 

 snow. The reindeer and musk oxen, which cannot conceal themselves in 

 this way, seek quiet localities and crowd tightly together. The exhaled 

 and transpired moisture of the herd forms a cloud above them, beneath 

 which the warmed air is retained as if in a closed room. 



The arctic avifauna is poor in land birds, though, as seen else- 

 where, marine birds, dependent on the sea for their food, are abundant. 

 All the smaller forms are migratory, as are many larger forms, such 

 as the swans and geese. The long summer days enable them to main- 

 tain a nearly constant search for food, and bring up their brood in a 



