410 LAND ANIMALS 



trees (martens, raccoons), less frequently warm, lined nests, such as 

 squirrels make for themselves. Only the larger mammals can brave 

 the winter cold without such protection, the deer, wolf, lynx, wild boar, 

 with the hare as the smallest, but these normally retire to sheltered 

 niches. Birds are better able to endure the cold of winter unprotected; 

 they have such excellent heat insulation that they require nothing 

 more than shelter from the winds. Even so, the ptarmigan in the far 

 north 119 and the capercaillie and black cock in Siberia, 120 the bobwhite 

 and others, dig holes in the snow and seek their food at the surface of 



the ground. 



The heat-retaining devices are furthermore reinforced for the 

 winter. In autumn, the mammals acquire their winter fur, thicker 

 than the summer coat, and at the beginning of the warm season this 

 is again lost; among birds the coat of feathers becomes thicker during 

 the fall molt. In addition to this, especially among the herbivores, a 

 thick layer of fat is deposited as a result of good nourishment during 

 the summer and fall. In the Spitzbergen reindeer, the subcutaneous fat 

 attains a thickness of 50-75 mm., in the Scandinavian reindeer it is 

 much thinner. 121 It is significant that among the animals of the cold 

 zone the fat is softer and less easily hardened than that of the 

 inhabitants of the warm regions. These masses of fat also serve as 

 food during the lean period of the winter, and the animals become 

 very thin toward spring. 



Scarcity of nourishment is a greater peril to the homoiothermal 

 animals than the cold. Small birds such as the titmice and the kinglets 

 with their relatively high food requirement can find sufficient food 

 only by continuous search throughout the short winter days (compare 

 Lapicque's experiment on Estrilda, p. 391) ; if the day is shortened 

 because of cloudiness many of them starve. Certain birds and mammals 

 of the temperate and cold zones gather supplies for the winter, during 

 the favorable period. The titmice and the nuthatch (Sitta) 1 - gather 

 seeds; the Old World jays (Garrulus) hide hazelnuts and acorns in 

 the cracks of trees and under leaves for later use. Mammals are espe- 

 cially given to storing provisions. Hamsters (Cricetus) , chipmunks 

 (Tamias), and ground squirrels {Citellus) collect grain and other food 

 in subterranean chambers, the hamster at times up to 25 kg.; dormice 

 and squirrels collect nuts, and voles {Microtus terrestris) gather roots 

 into their nests as winter food. Such vegetable materials keep well if 

 they are stored in a dried condition. The arctic fox even manages to 

 store meat by placing ptarmigan or similar booty in crevices in the 

 ice; the polecat {Putorius putorius) is said to paralyze common grass 

 frogs by a bite in the spinal cord and bring them together in a cavity 



