EVOLUTIONARY ASPECTS IN MAMMALS 



hibernating season when the animal is in a transitory state. When 

 only body temperatures from representative times of activity are 

 taken, Herter (1934) reports a range of 2 C, and reports a dif- 

 ference in activity temperature between summer and winter of 

 about 1 G for the hedgehog. The one specimen of Erinaceus 

 europeus subjected to measurements by Morrison (1957), shows 

 the range of 34.8 C to 36.4 C with an average of 35.6 C. Although 

 it has been decided that we leave out any detailed discussion of 

 hibernation in this symposium, it seems justifiable to comment on 

 Eisentraut's statement (1956a) that hibernators generally show 

 imperfect heat regulation also during the active season; and he 

 includes all known hibernators in the group of lower warm-blooded 

 animals. I am, myself, and I know many others are, willing to 

 challenge this statement. "Hibernation," although used to express 

 the seasonal and diurnal condition of sleep displayed by a variety 

 of the smaller mammals is really not representative as a term 

 for all these species, and a great number of the true hibernators 

 can, in my mind, be classified as extremely specialized and very 

 far from primitive in their mode of temperature regulation. 



The shrews represent an interesting and successful group of 

 Insectivores. In spite of their small size, which is obviously dis- 

 advantageous in the cold, they have left behind most fellow Insecti- 

 vores and invaded the north temperate and even arctic regions. 

 They are, for instance, found here aroiind Fairbanks and in Norway 

 even farther north. Apparently, no detailed studies have been made 

 of these extreme northern populations of shrews. Morrison, Ryser, 

 and Dawe (1959) have, however, presented a careful study of the 

 shrew (Sorex cinereus) obtained from the Wisconsin region. The 

 authors noted that the manipulation and handling of the animals dur- 

 ing measurements of their body temperatures invariably increased 

 the body temperature. The increase occurs very rapidly, often 

 1 G per minute, and the body temperature could attain a level 

 above 41 G. Similar elevation in body temperature became apparent 

 after exercise. The body temperature seems always to level off 

 at about 41 G. Figure 8 shows the results. More than anything it 

 demonstrates a great variability of body temperature; this animal 

 is unquestionably one of the more successful small animals living 

 today. Its labile body temperature can hardly be classified as a 



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