ADAPTATIONS TO HIGH ALTITUDES 49 



also is exhausting. Now it is very interesting to com- 

 pare the heart of the Ptarmigan from high altitudes 

 with that of the Willow Grouse from low levels and 

 to find that the heart of the Ptarmigan is specially 

 strong. 



Everyone who has climbed a little in the Alps has 

 heard the shrill whistle of the Marmot (Arctomys 

 marmotta) a danger signal that is followed by 

 instantaneous disappearance. The animal is about 

 the size of a wild rabbit, greyish yellow and brown in 

 colour, and of sociable disposition. It is a rodent and 

 a vegetarian. When winter comes on the Marmots 

 retire in companies of a dozen or more to their bur- 

 row, which has been previously stocked with dried 

 grass. This is largely for bedding and blankets, for 

 survival depends on keeping up a certain degree of 

 warmth in the retreat, but it may also serve to some 

 extent as provender. The Marmot's adaptation to the 

 mountains is to sink into winter sleep, and this 

 deserves our study. 



Birds and Mammals are alone among living crea- 

 tures in being "warmblooded," which means that they 

 are able to preserve an almost constant body tempera- 

 ture year in, year out. In an intricate way, which we 

 cannot explain shortly, they are able to adjust the pro- 

 duction of animal heat within their bodies to the loss 

 of heat by the skin. In other backboned animals 

 namely, Reptiles, Amphibians, and Fishes there is 

 no such power, and the body temperature tends to 

 approximate to that of the outside world. This is 

 what is meant by "coldblooded." When the outside 

 temperature is too low for a continuance of the 

 ordinary vital processes, these coldblooded creatures 



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