ARCTIC METABOLIC ECONOMY 335 



row as will conserve their feeble production of metabolic heat. For 

 the very numerous arctic population of ground squirrels, hibernation 

 is a successful evasion of winter cold, but as far as I know the faculty 

 of hibernation at depressed body temperature is only exercised in the 

 Arctic by two mammalian species, Citellus and Arctomys. 



It has been suggested that ptarmigan hibernate in their burrows 

 under the snow, but no indication was found that the body temperature 

 of willow ptarmigan shot early in winter mornings or late in the 

 afternoon differed from the midday level (L. Irving and J. Krog, 

 1954) . Willow ptarmigan enter snow burrows at evening with crops 

 containing tips of willow twigs and buds amounting to as much as 

 one sixth their body weight. When they leave in the morning a large 

 mass of droppings shows that their digestion has been active during 

 the night. 



Birds of the tundra lack the shelter of trees. Cliff dwelling gyr- 

 falcons and ravens on their roosts can find shelter from wind, but I 

 doubt if they find or require protection from arctic cold. Cade (1953) 

 has observed the common redpoll in winter near Fairbanks entering 

 apertures formed through the snow around weed stems, and it is 

 reasonable to suppose that such shelters afford some protection. 

 Simon Paneak remarked that hoary redpolls had been seen entering 

 the spaces in the snow through which willow bushes protrude. Inas- 

 much as these openings are sealed by each fresh snow and do not com- 

 monly form before the sun rises in winter, we did not think that the 

 redpolls would regularly be able to find shelter much warmer than in 

 the open air. I do not know where chickadees, jays, and grosbeaks 

 roost on the tundra, but I do not think that any small tundra birds use 

 shelters so well enclosed that at night the air in them would be warmed 

 much above the temperature outside. Under these conditions arctic 

 birds cannot reduce their body temperature by hibernation or in tor- 

 pidity, for because of the diminished heat production observed in 

 torpid birds they could not be expected to counteract the cold of even 

 just freezing weather (Pearson, 1953). Arctic tundra mammals 

 smaller than hares remain under the snow. Signs of pursuit by fox 

 and weasel reveal the extensive winter activity of the great numbers of 

 mice and lemmings otherwise concealed by snow. Nordenskiold (1882, 

 p. 114) described vividly how the melting of the snow disclosed the sur- 

 prising extent of the hidden winter activities of arctic small mammals 

 by showing the runways, nests, and dung. We have no measurements 

 of the body temperature of these small mammals in their natural winter 

 habitat, but the indications are that they are those usual for mammals. 

 Underneath the snow the small mammals are exposed only to modi- 

 fied arctic cold, but the small arctic birds seem to be unable to escape 

 the severe cold of arctic winter. 



