DAWSON, W. R. 



appears to be controlled by a center in the midbrain, judging by von 

 Saalfeld's observations on Rock Doves ( Colum ba livia) , and cannot 

 be elicited by peripheral stimulation (Randall, 1943). Pan ting is un- 

 affected byvagotomy inthe rockdove.butis abolished by this opera- 

 tion in the Domestic Fowl (Hiestand and Randall, 1942). 



Whatever the details of the original and present features of the 

 mechanisms governing temperature regulation in birds, it is appar- 

 ent that they must have been dependent fundamentally on a capacity 

 for the detection of absolute temperature (as opposed to detection 

 of temperature change) . This capacity is not an original develop- 

 ment by homeotherms, but also must be present in many poikilo- 

 ther ms , judging by the widespread distribution of temperature selec- 

 tion among them (Fry, 1958). The functional basis of absolute tem- 

 perature detection is little understood, although analysis of the non- 

 adapting fraction of the thermal sensitivity of some peripheral re- 

 ceptors is providing some information (Bullock, 1955). The means by 

 which it is accomplished in the behavioral regulation of body tem- 

 perature by reptiles is unknown, but recent work (DeWitt, personal 

 communication) suggests that in the lizard ( Dipsosaurus dorsalis) . 

 and presumably in other species, it is actually the temperature of the 

 brain or one of its parts thatis regiilated. Rodbard (1948) claims to 

 have demonstrated the existence of a thermally sensitive area in the 

 hypothalamus of the turtle, which controls blood pressure, and on 

 this basis suggests that the thermoregulatory centers of homeo- 

 therms evolved from a hypothalamic area controlling circulatory ac- 

 tivity. Such a conclusion seems premature considering the absence 

 of information on the neural mechanisms responsible for controlling 

 thermoregulatory behavior and panting in reptiles. 



Metabolic Level and Chemical Regulation 



The basal metabolic rates of birds and mammals are as much as 

 eightfold greater than the resting metabolic rates of reptiles of com- 

 parable size at the same body temperature (Martin, 1903; Benedict, 

 1932, 1938; and Daws on and Bartholomew, 19 58), and the intensifica- 

 tion of metabolism has apparently comprised a most important step 

 in the evolution of homeothermy. Zeuthen (19 53) has suggested that 

 this intensification was achieved through prolongation of the develop- 

 mental phase in which metabolism and size are nearly proportional. 



52 



