390 



LAND ANIMALS 



North America. Jackrabbits of the subgenus Macrotolagus, in the 

 southwestern United States, have ears that measure almost a third of 

 the body length; in the common jackrabbit (L. campestris) , from 

 Kansas to Canada, ears are as long as heads; among the varying hares 

 (L. americanus) , that extend southward as far as 40°-45° N. latitude, 

 the ears are shorter, and still shorter among the arctic hares (L. 

 arcticus) . The size difference of the ears of the desert fox (Canis 

 zerda) , of the European fox (C. wipes) and of the polar fox 

 (C. lagopus) is shown in Fig. 112. In Siberia, the ears of the wild 

 hogs, the red deer, the roe deer, the fox, and wildcat are relatively 

 smaller, often positively smaller, than those of the smaller German 

 forms. 48 Figure 113 exhibits the extreme difference in the surface de- 



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t\ 



-<*> 





■&' 



Fig. 112. — Head of arctic fox (Canis lagopus), a; red fox (Canis vulpes), b; and 



desert fox (Cayiis zerda), c. 



velopment and bod}' form which may be found in antelopes from cold 

 and warm climates. Similar differences appear in Gazella picticauda, 

 of the Himalayas at an elevation of 4000-5000 m. above sea level, 

 and G. bennetti, inhabiting the plains of north and central India; with 

 approximately equal length, the mountain animal has shorter legs, 

 ears, and tail. 49 



The body appendages of many mammals are lengthened by the 

 direct action of various factors. Mice reared at higher temperatures 

 developed relatively longer ears and feet. The growth of lop-ears of 

 rabbits is favored by leaving the animals in hutches with a temperature 

 of 15°. Such considerations make it possible that the body propor- 

 tions just discussed are, at least in part, a direct consequence of the 

 temperatures to which the animals are exposed in nature. 



Heat given off by radiation must be restored by an increase of 

 metabolism. The reduction of food into small bits by the gizzard of 

 birds or by the teeth of mammals accelerates the digestion of food- 

 stuffs and thereby makes possible greater intake of food; this forms 

 the basis for the increased body temperature. An increase of the body 

 temperature, in turn, causes the acceleration of digestion, and this 



