384 LAND ANIMALS 



with the reduction of heat radiation and the increase of heat pro- 

 duction; in the last case, the regulative processes are reversed. 



Heat radiation through the surface is diminished by thermal in- 

 sulation. The poorest conductor of heat and hence the most effective 

 insulator is air. The "warming" effect of the feathery and hairy coats 

 of the birds and mammals is due to the fact that these coats retain an 

 encircling layer of air immediately about the body. In birds the coverts 

 or contour feathers cover thin, air-retaining down; in mammals the 

 bristly outer hairs close over the inner woolly hairs and lie thickly 

 upon them, attracted by the opposite electrical charge of the lower 

 layers, 22 so that in both a layer of air is held between the down or 

 wool. The birds have in addition an inner air envelope in the form of 

 their air chambers. In both birds and mammals, the poor heat- 

 conducting qualities of the subcutaneous fat add further insulation. 



Birds are superior to the mammals in cold resistance; they do not 

 possess richly vascular exposed structures such as ears and tail; their 

 lower legs and feet are tendinous and the borders of their mouths are 

 non-vascular and horny, and thus give up little heat. Through the 

 application of the wings to the body and by other protective devices 

 already mentioned, they possess exceptional means of retaining warmth ; 

 further, their body temperature is higher than that of mammals. For 

 these reasons it is not surprising that they endure the cold of winter 

 much better than do mammals. The smallest mammal that is fully 

 exposed to the winter temperatures of north Germany is the hare; all 

 the smaller and many larger mammals, mice, shrews, moles, squirrels, 

 martens, foxes, etc., retreat to earth or holes in trees, or to well-lined 

 nests. Small birds like the wren and titmouse do not need such 

 protection. 



The thickness of hair and feather coats is definitely related to en- 

 vironmental conditions Among birds, the fliers that rise to the higher 

 and colder air strata have a thicker plumage than those remaining close 

 to the ground in the same climate; in the birds of prey, the raven, 

 and the passerine birds, feathers compose 10-13% of the body weight, 

 the gallinaceous birds, poultry, pigeons, and web-footed birds, only 

 7-9 %. 23 In cold climates the hair and feather coats are thicker than 

 in warmer climates. Reindeer and Tibetan antelope which are exposed 

 to cold winds have a rough, coarse-haired, thick pelage, whose bristly 

 hairs are so thick and matted together that the strongest blizzard 

 cannot blow them apart. The most valuable furs come from the coldest 

 regions. The thickness and consequently the value of fox or skunk fur 

 is graded according to its origin. The Manchurian tiger (Felis tigris 

 amurensis) has a very thick, shaggy coat in contrast with the Bengal 



