PBOSSER 



some birds and mammals can function over a much wider range of 

 temperature than can the core tissues. The corresponding enzymes 

 must differ in cold functional skin and in constantly warm liver. 



When the ambient temperature (air or water) falls, a homeo- 

 therm shows a typical sequence of protective responses. The meta- 

 bolic response in relation to body temperature is diagrammed in 

 Figure 1. Peripheral cold receptors signal the drop in skin tempera- 

 ture and initiate reflexes such as hair or feather erection, peripheral 

 vasoconstriction, and behavior such as huddling. These initial re- 

 sponses result inheatretentionby increased insulation. With further 

 chilling, the temperature- sensitive center in the hypothalamus is 

 stimulated and further defenses maybe mobilized. Cooling below the 

 critical ambient temperature or to that temperature below which in- 

 sulative changes are inadequate so that a transient drop in body tem- 

 perature occurs, results in increased metabolism which serves to 

 maintain body temperature. Nor- adrenaline secretion is enhanced, 

 and shivering maybe initiated and heat production increased. If cold 

 stress continues, the hypothalamus activates the anterior pituitary 

 to liberate adrenocorticotropic and thyrotropic hormones. The ad- 

 renal cortex and thyroid initiate a metabolic increase and extensive 

 biochemical responses of various organs, particularly liver to vary- 

 ing extents in different species. In laboratory acclimation to cold 

 some animals show an increased standard metabolism; in field ac- 

 climatization many animals show a reduction in critical temperature. 

 Some metabolic enzymes become more active than others and the 

 sensitivity to stimulating hormones is altered (Hannon, 1960; Hart, 

 1957; Heroux, 1960). The increased metabolism of cold acclimation 

 may persist after withdrawal of hormonal stimulation, and in the 

 annual cycle of winter, insulative adaptations make metabolic ones 

 less necessary. In some species the adrenal cortex is active and 

 the thyroid less active under natural winter conditions. 



Over a thermoneutral zone the insulative changes are sufficient 

 to maintain relativeconstancyofbody temperature in a homeotherm. 

 At elevated ambient temperatures, reflexes provide increased peri- 

 pheral bloodfloWjSurfacecoolingbysweat, panting, and other means 

 of controlling body temperature. However, there is no reduction in 

 metabolism and in conditions of fever the oxygen consumption may 

 increase. 



