ANIMAL LIFE OF POLAR REGIONS 511 



maritima, of Spitzbergen, has accustomed itself to a plant diet. The 

 polar bear eats fishes and birds' eggs and even plant food when seals 

 are unavailable. The arctic fox is still more completely omnivorous 

 and feeds on mollusks and other sea food. Even the reindeer eats algae 

 from the coast and occasionally takes a lemming. 16 Some mammals 

 lay in food supplies for winter. The lemmings Dicrostonyx torquatus 

 and hudsonius dig out hollows under stones in which they store root- 

 lets, and when there is a surplus the arctic fox stores ptarmigan and 

 other food in ice crevices. 



The tundra in many places is riddled with lemming holes, and the 

 lemmings are, in many ways, the most joyous feature of the country. 

 During the summer, with the activity of rabbits, they pop in and out 

 of their holes. The nests are made in grass and moss, and young can 

 be found as late as September. A full-grown lemming is about 6 inches 

 long from nose to the tip of the short rat-like tail. Lemmings are key 

 industry animals, and as such their well-established but unexplained 

 cycles of abundance, with greatest numbers every three or four years, 

 greatly affect the numbers of their associates. 



Reduction of surface is an important means of heat conservation in 

 arctic animals. This may take place by development of a compact 

 form, with reduction of the appendages (especially the ears and tail), 

 or by increase in size in accordance with the Bergmann Rule (see 

 Chapter XX) . The musk ox is a typical example of this type of body 

 form. Its legs are so short that it stands only 1.1 m. high, though 

 2.5 m. long. The neck is thick, the tail only 7 cm. long, and the external 

 ears almost entirely concealed in the furry coat. 



The warm-blooded animals of polar regions contrast further with 

 the poikilothermal forms in their pale or pure white coloration. The 

 poikilothermal forms are almost all dark, and thus absorb the greatest 

 possible amount of heat during the brief season of their activity. The 

 white coloration of the homoiotherms radiates less heat than the dark, 

 and prevention of heat loss is evidently of greater importance to them 

 than absorption of the relatively small amounts of heat received from 

 the sun. 



The earlier opinion, that the white coloration of polar animals is 

 a concealing coloration, which makes both prey and predators invisible 

 on the snowy landscape, need not be discarded in all cases. The 

 ptarmigan, for example, keep to the remaining snow patches in spring 

 until they lose their white winter plumage. 17 This factor does not 

 apply to the Greenland falcon or the snowy owl, since they strike their 

 prey from above, so that their pale coloration does not conceal them, 

 and they are themselves without enemies from which they require 



