NO. 12 BODY TEMPERATURE OF BIRDS — WETMORE 29 



of value not only in overcoming cold but in enabling the organism to 

 withstand excessive heat. 



It seems probable that in our living fishes there is little actual tem- 

 perature control. In Amphibia, this regulation is developed to some 

 extent, and it has progressed somewhat farther in modern reptiles. 

 In the bird, however, the regulation of body temperature has reached 

 its highest point, though birds stand second to mammals from an 

 evolutionary standpoint. Proof of this is found in the fact that birds 

 have the highest body temperatures known, and that none of them 

 hibernate (in spite of ancient beliefs to the contrary). Where con- 

 ditions become too unfavorable, birds, through their power of flight, 

 pass readily to regions where the environment is more clement. They 

 are enabled, therefore, to foster their powers of temperature control 

 and keep them at the highest pitch. Small mammals, on the contrary, 

 are more or less sedentary and in many cases must still undergo 

 hibernation in order to maintain themselves in regions with cold 

 winters. As they must always hibernate in order to survive there is, 

 in their case, less incentive to develop temperature and temperature 

 control beyond a certain point. 



Thermogenic centers or areas in the central nervous system devel- 

 oped for temperature control have been studied in mammals and have 

 been fixed tentatively by some in or near the corpus striatum. Others 

 would recognize a cortical heat center. Seemingly this matter has 

 received little attention in birds and it would be unwise in the absence 

 of definite data to decide that this function is vested in the same areas 

 in this group when the wide separation between birds and mammals 

 is considered. It may be assumed as certain that heat production and 

 heat control are under nervous direction and that these two functions 

 are directly concerned in whatever mechanism has developed for 

 temperature control. 



The origin of the warm-blooded animal may be attributed to natural 

 selection in which certain individuals showed a slight reaction against 

 temperature conditions producing hibernation in their fellows. In 

 other words, these favored ones were able to remain active in a tem- 

 perature a few degrees colder than others of their kind. With this 

 tendency as a basis and with strains developing in which this tendency 

 was perpetuated it followed that there were evolved groups of species 

 with a more independent metabolism in regard to the degree or the 

 lack of heat of the surrounding medium. " Warm blood " therefore 

 arose in a struggle against enforced hibernation. During evolution 

 of the vertebrates it may be that among living groups of today, warm 



