Temperature: MetahoUc Aspects and Perception 349 



of the stress-regulating mechanism oF the adrenal cortex. Other encloerines, 

 particularly the thyroid, may also be concerned in acclimatization. Rats ac- 

 climatized to cold synthesize and store large quantities of ascorbic acid in 

 various tissues, particularly in the adrenals, and guinea pigs are able to endure 

 cold only if fed massive amounts of ascorbic acid.'" ^lan shows temperature 

 acclimatization, as when in a hot desert or tropics, but he cannot acclimatize 

 to dehydration.^ Most arctic birds and mammals have more insulation than 

 tropical ones, and individuals of the same species vary in fur coat according 

 to the temperature at which they live; the fur is heavier on rats and dogs 

 maintained at low temperatures than on those maintained at high temperatures. 

 The metabolic changes occurring in acclimatization have not been identi 

 fied.'*' The melting point of fats deposited in animal bodies at high tempera- 

 tures is higher than that of fats deposited at low temperatures. At low tem- 

 peratures predominantly unsaturated fats are formed. Acclimatization may 

 well be associated with lipid changes. Older suggestions of differences in 

 water content seem not to hold. There may also be changes in the temperature 

 response of certain enzymes. The inactivation temperature for amylases, 

 pepsin, and trypsin from fish is lower than for the same enzymes from mam- 

 mals.-^^- ^^''^ The oxygen consumption (at a given temperature) of brain tissue 

 from a polar cod is higher than that of brain tissue from a black bass, and 

 below 10'' changes in temperature alter respiration of the bass brain more than 

 they do that of the polar cod brain. ^^ Examination of optimal (critical) 

 temperatures and of temperature characteristics for a variety of enzymes from 

 organisms acclimatized to high or low temperatures might indicate changes in 

 critical enzyme proteins. 



TEMPERATURE RELATIONS IN POIKILOTHERMIC ANIIVIALS 



In poikilothermic animals the factors fa\oring loss of heat tend to equal 

 the factors producing body heat, and the body temperature approaches en- 

 vironmental temperature. However, small thermal gradients exist from regions 

 of active oxidative metabolism toward the body surface, and mean body tem- 

 perature fluctuates with muscular activity. The temperature relations of 

 poikilothermic animals are different according to whether thev live in water, 

 in moist air, or in dry air. In water the heat loss is by conduction and con- 

 vection; in moist air the evaporative loss is less than in dry air. Water tempera- 

 tures are more constant than air temperatures; hence aquatic animals are less 

 subject to rapid gain or loss of heat. 



Aquatic Poikilotherms : Body Temperature. Aquatic poikilotherms follow 

 changes in environmental temperature rapidly and precisely. Loss of heat 

 cannot occur by vaporization (except in air breathers), and in most aquatic 

 animals there are no mechanisms for actixely reducing the heat loss which 

 normally occurs by conduction and conx'cction. Some older measurements of 

 body temperature of aquatic animals indicate \alues higher than the environ- 

 mental temperature, but more recent data show comjilete conformance with 

 the environmental temperature. '"'•' Certainly in small aquatic animals heat 

 conduction is rapid, metabolic rate low, and mean body temperature the same 

 as water temperature. In larger animals, such as large fish, it is probable that 

 during active swimming the temperature of the muscles may be appreciably 

 higher than the skin temperature. The metabolism of several species of fish 



