EVOLUTION OF TEMPERATURE 

 REGULATION IN MAMMALS 



Kjell Johansen 



The term "evolution" has a special affinity to allofus.lt 

 is an integrating symbol of what we are all coxicerned with. How- 

 ever, most of us, I am sure, are somewhat reluctant to use the 

 word in our scientific work. One reason for this, at least among 

 physiologists, is that our section of biology is founded solidly on 

 measurements and carefully designed experiments, and the infor- 

 mation acquired does not readily fall into line with the observations 

 that have formed the theories of evolution. Fossils, unfortunately, 

 do not render themselves easily to physiological study. I have a 

 strong interest In the possible routes along which physiological 

 mechanisms may have evolved. In my near awe for the term "evolu- 

 tion" I have found it expressive and also comforting to myself to say 

 that I have an interest in the physiological phylogeny of certain func- 

 tions. This term can be applied only to information compatible with 

 the exactness required in a physiological study. Moreover, by using 

 the phylum in a comparative manner, we are approaching the home 

 grounds of evolution. As you can see from the program, Dr. Dawson 

 and I have been ascribed the rather doubtful task of discussing the 

 evolution of oneof the profound and striking physiological character- 

 istics of thehighervertebrates,homeothermy.Today afew mamma- 

 lian forms exist which retain a number of extinct morphological 

 characters. These animals are often called living fossils or missing 

 links, and are mainly represented by the Australian monotremes and 

 marsupials. For many reasons, I have decided to confine the main 

 parts of my discussion to these orders plus the New World marsupi- 

 als and Xenarthra, which also represent the archiac forms rather 

 well, in spite of their extreme specializations. Reference will also 

 be made briefly to other orders of mammals classified among the 

 more primitive forms. These are the Insectivores and the Chirop- 

 tera, but there will be no time to discuss phylogenetic implications 

 of the evolvement of temperature regulation within a distinct order, 

 like for instance, the rodents. Brief digressions will be made to 

 exemplify how and why environmental extremes may lead to evolve- 

 ment of specialized physiological mechanisms also in the higher 



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