Temperature: Metabolic Aspects and Perception 369 



too high, a fever begins by shivering, piloerection, cutaneous vasoconstriction, 

 and absence ot sweating; heat is retained, and the body temperature then rises. 

 If the normal body is cooled, as by drinking ice water, or if it is warmed, the 

 regulatory mechanisms bring the body temperature back to "optimum."-' 



Chemical Mechanisms of Temperature Regulation. The chemical or 

 metabolic factors in temperature regulation are important in two respects: 

 responses to temperature change, and basal metabolic level. -^'^ 



As air temperature becomes lower or higher the metabolism remains con- 

 stant, while the physical insulating mechanisms, thickness of body coat and 

 vasomotor reactions, compensate for changes in thermal loss. Finally these 

 mechanisms are insufficient, and at elevated blood temperature metabolic rate 

 rises according to tissue temperature, whereas at depressed blood temperature 

 the thermoregulating center is stimulated to activate muscular movement (as 

 shivering) and probably secretion of endocrines which result in increased 

 tissue metabolism (heat production). That is, metabohsm of homoiotherms is 

 minimal in a range of thermal neutrality and increases both above and below 

 this range, whereas in poikilotherms metabolism varies directly with tempera- 

 ture. 



There is much species variation in the metabolic response to low tempera- 

 ture. In the English sparrow, for example, the body temperature was 41.5° C. 

 for air temperatures from to 37°, yet the heat produced per gram per hour 

 at 0° was double the amount produced at 37°; above 37° the body temperature 

 rose somewhat and heat production increased more.'*'* When a mammal is 

 chilled there is increased liberation of adrenin, also increased production of 

 adrenal cortical hormone and thyroid hormone, hence increased metabolism. 

 Adrenalectomized or thyroidectomized rats lose ability to compensate for. 

 cold.-"" The hypophysis may also be involved. Thyroid activity is enhanced 

 at low temperatures.'^^ Man in water at 20° C. produces heat at 3 to 6 times 

 the basal rate, and by this means keeps his rectal temperature from going below 

 35° C; in water at a temperature lower than 20° C, heat production decreases 

 and the body temperature drops. ^-^ 



Birds and mammals of the arctic have better insulating ability than tropical 

 species. This is indicated by the extent of the temperature range over which 

 metabolism is constant, that is, over which the insulating reactions are suffi- 

 cient for maintenance of body temperature. In the arctic white fox, for exam- 

 ple, air temperature can be lowered to — 30° C. without any increase in 

 metabolism; cold stress, however, is sufficient to elicit a rise in metabolism in 

 the Alaskan ground squirrel at — 10°, in lemmings at 15°, and in polar bear 

 cubs at 0° C. In sloths, night monkeys of Panama, or jungle rats, metabolism 

 increases when the air temperature falls to about 25° C. (Fig. 98). ^•''^ As the 

 air temperature falls, heat production increases by a small amount in well 

 acclimatized birds and mammals of the arctic and by greater amounts in 

 tropical species. In all of them, however, the slope of the curve of heat pro- 

 duction extrapolates back to zero at the normal body temperature (Fig. 98). 

 If an animal is unable to maintain its body temperature by its insulating and 

 metabolic reactions, heat production then falls off with chilling, and long 

 survival is impossible. 



The metabolic response to stress of cold can be altered by acclimatization. 

 For example, rats kept for 3 months at 0° to 2° maintained a constant tempera- 



