between May and September. The white winter color 

 apparently does not give special protection against 

 heat loss from the body, as has sometimes been 

 thought (Hammel 1956). 



A major habitat problem that tundra animals 

 must solve is tolerating or avoiding the long severe 

 cold of the winter season. Cold-blooded animals are 

 generally acclimatized so that they remain active at 

 temperatures down to freezing much better than their 

 relatives in temperate and tropical zones. This is 

 particularly true for aquatic species (Scholander et 

 al. 1953, Bullock 1955). Invertebrates commonly 

 pass the winter in the larval or pupa! stage that is 

 especially resistant to freezing, although beetles, 

 spiders and some other forms may overwinter as 

 adults. Rotifers, tardigrades, midge fly larvae, and 

 dytiscid and hydrophilid beetles may be frozen in the 

 ice for months or even years, yet resume activity 

 immediately on thawing (Lindsey 1940, McClure 

 1943, Andersen 1946). Because of the short growing 

 season and slow development, many tundra insects 

 require two or more summers to complete their de- 

 velopment. 



The larger mammals and over-wintering birds 

 have good insulation in long, dense pelage or plum- 

 age, and in fat. Heat production in their bodies is 

 not greatly increased until very low air temperatures 

 are reached (Scholander et al. 1950). The tarsi and 

 legs of ptarmigan and snowy owls become well 

 feathered in the winter. 



Voles, lemmings, and ermines escape the winter 

 cold by staying in their runways and nests under the 

 snow. The ptarmigan also digs tunnels into snow- 

 banks where it roosts protected from the cold (Wet- 

 more 1945), sometimes for days at a time. Only the 

 arctic ground squirrel truly hibernates. This it does 

 by excavating burrows into sandbanks or hills which, 

 because of exceptional drainage, possess an area that 

 remains unfrozen between the deep permafrost and 

 the winter frost at the Surface (Mayer 1953). The 

 bears den up during the cold weather but remain ac- 

 tive to the extent of giving birth to their young in 

 the middle of the winter. 



Those species that cannot tolerate or escape the 

 winter cold and lack of food migrate. The barren- 

 ground caribou on the mainland migrates in long 

 strung-out armies to the southern portions of the 

 tundra, even well beyond the tree-line into the forest- 

 tundra, to pass the winter, and their trails remain con- 

 spicuous throughout the year (C.H.D. Clarke 1940, 

 Harper 1955). The caribou on Greenland, Spitz- 

 bergen, and the northern islands of the Canadian 

 Archipelago are necessarily resident throughout the 

 year. Migration of the bird fauna in this biociation 

 is nearly complete : during the winter only an occa- 

 sional hawk, ptarmigan, raven, or owl will be en- 

 countered over the land, although marine birds occur 



wherever there is open sea. When the birds return 

 in the spring they are quick to get nesting started. 

 Often they are already mated, carry through their 

 nesting cycle quickly, and then leave promptly again 

 for southern latitudes. 



In general, the melting of the snow and the break- 

 up of ice is the signal of transition from winter to 

 summer. Although it is possible to recognize all of 

 the four aspects (Sjzirensen 1941), the change from 

 winter to summer and back again to winter is so 

 rapid that all aspects are abbreviated except the 

 hiemal and aestival. May is the usual month of par- 

 turition among the larger mammals, and the peak of 

 bird nesting comes in late June and early July. 



Owls, hawks, water birds, and some passerine 

 species do not breed in those years in which scarcity 

 of food or delayed freeing of nesting grounds of snow 

 and ice are detrimental to survival. Failure of 

 hawks and owls to breed occurs especially in years 

 when the lemming population is low (Marshall 

 1952). 



Ptarmigan, hare, voles and lemmings, and their 

 predators, particularly the fox and snowy owl, are 

 subject to oscillations in abundance that come at 

 intervals of either 3-4 or 9-10 years. These oscilla- 

 tions are more pronounced in the arctic tundra than 

 in any other biome. 



The food coactions of the herbivorous animals are 

 of interest. Caribou feed on lichens, including rein- 

 deer moss, especially in the winter, which they un- 

 cover by pawing through the snow. During the sum- 

 mer they also consume shrubby growth and sedges 

 (Harper 1955). Grass is the principal food of the 

 muskox, but it also eats willow brouse and, less fre- 

 quently, lichens and mosses (Jackson 1956). Ptar- 

 migan feed on plant material left behind by these 

 large animals. The chief food of ptarmigan, how- 

 ever, is the buds, leaves, and tender branches of 

 willow and other shrubby plants not easily obscured 

 by snow. Their winter food appears to be richer in 

 fats and contains less protein than the food they con- 

 sume during the summer (Gelting 1937). Gulls are 

 known to eat warble fly larvae rising from the skin 

 of caribou (Scalon 1937) but depend mainly on dead 

 fish that they find in open water, or on carrion. Most 

 of the small passerine birds are seed-eaters or mixed 

 seed- and insect-eaters. Berries become abundant in 

 August, and are much sought after by birds. Exclu- 

 sively insectivorous land birds, such as warblers, 

 would find great difficulty in surviving and reproduc- 

 ing on the open tundra. Shorebirds are largely in- 

 sectivorous but depend largely on aquatic forms for 

 food. 



Although the continuous arctic summer light 

 permits activity throughout the twenty-four hour 

 day, most mammals and birds need periods of rest, 

 which they take at any time. Birds appear to rest 



320 Geographic distribution of communities 



