EVOLUTION OF AVIAN TEMPERATURE REGULATION 



Hemmingsen (1960) emphasizes that the transition from a poikilo- 

 thermic to a homeothermic metabolic level was to a great extent de- 

 pendent on an increase in the area of the respiratory surfaces. Ob- 

 viously this transition exercised a profound effect on the respiratory, 

 circulatory, and other organ systems of the nascent homeotherms. 

 The bolsteringof the capacity of these systems, which served to sus- 

 tain heightened demands of metabolism, probably contributed subse- 

 quently to the development of the thermoregulatory processes. For 

 example, modifications of the cardiovascular system, which allowed 

 operation with a higher cardiac output and higher systemic blood 

 pressure, probably improved capacities for transport of heat over 

 those possessed by reptiles. 



The elevation of the general level of metabolism made possible 

 the development of effective chemical regulation. Such regulation 

 appears to provide the initial means by which young birds control 

 body temperature in moderate to cool environments. For example, 

 the development of temperature regulation in young House Wrens 

 (Troglodytes aedon )at an ambient temperature of 18 C is closely 

 correlated with the appearance of muscle tremors (Odum, 1942). 

 Similarly, in young domestic fowl, the ability to maintain body tem- 

 perature at a high level during exposure to an ambient temperature 

 of 20 C initially appears to be associated with the acquisition of the 

 ability to shiver (Randall, 1943). These observations suggestthat the 

 development of chemical thermoregulation was one of the initial 

 steps in the evolutionofhomeother my in birds. Martin (1903) reach- 

 ed a similar conclusion for the evolution of this condition in mam- 

 mals on the basis of his studies of temperature regulation in adult 

 monotremes, marsupials, and placentals. If this suggestion is cor- 

 rect, the advent of chemical thermoregulation must have provided 

 the initial means by which a level of body temperature established 

 under favorable conditions as a result of suitable behavioral pat- 

 terns and of an intensified level of metabolism could be maintained 

 in cooler surroundings. 



The principal development in the evolution of chemical thermo- 

 regulation in the avian line has concerned mechanisms for varying 

 muscular heat production. Increasing muscle tonus and, ultimately, 

 shivering are the principal means besides activity by which con- 

 temporary birds augment their heat production (Steen and Enger, 

 19 57; King and Farner, 1960). The ability to sustain elevated levels 



53 



