346 



Comparative Animal Physiology 



25-27° C, but some reflexes remain at lower temperatures, and death results 

 in a cat at 16° C. In a guinea pig death results at 14° C.-^ When men are 

 immersed in water heat production fails to keep up with heat loss below a 

 water temperature of 20° C. Death occurs after less than an hour's duration 

 of temperature of 26.5° C. (taken rectally).^-^ Similarly, dogs lose conscious- 

 ness at a rectal temperature of 22.5-26° (cerebral 24.5-28° C.).^^ Hibernating 

 mammals can survive lower body temperatures than non-hibernating animals 

 (Table 59); hibernating bats die if vital organs are frozen, but opossums die 

 at temperatures a few degrees above that causing freezing. The primary causes 

 of death at low temperatures, but without freezing, are unknown. 



LETHAL EFFECTS OF ELEVATED TEMPERATURES 



The highest tolerable body temperature has been ascertained for many 

 animals and some representative values are given in Table 59. Numerous 

 mechanisms of heat death are suggested, and no one mechanism operates for 

 all animals.'^" Heat may kill by enzyme inactivation. As temperature rises, 

 enzyme activity increases, and destruction of the enzyme is also accelerated; 

 above a certain temperature, the enzyme is so rapidly destroyed that the net 

 effect is reduced activity.'"'" Many enzymes are inactivated above 35-45° C, 

 although higher temperatures are needed for protein denaturation. Heat may 

 also kill by irreversible protein coagulation. Cellular lipids also change their 

 physical state at elevated temperatures. Blowfly larvae reared at 12-18° have 

 a lower heat death temperature and have lipoids with more double bonds than 

 have larvae reared at 30-36°, yet two species with the same kind of fat die at 

 different temperatures; hence fat breakdown per se can hardly be the cause 

 of death."'^ Cell membranes at high temperatures become freely permeable. 

 In higher animals the heated tissues may liberate toxins which cause damage 

 at a distance."^ An old theory, that at high temperatures the oxygen supply is 

 inadequate, fails to hold for insects,^- although in vertebrates the afhnity of 

 hemoglobin for oxygen decreases significantly at high temperatures. Whatever 

 the mechanisms of heat death, they are certainly multiple and they are sub- 

 ject to alteration, as judged by resistance to heat. 



The high temperature lethal for an animal depends on duration of exposure 

 to the elevated temperature, and high lethal temperatures as in Table 59 are 

 meaningful only if exposure times are given. Also, when the temperature is 

 raised gradually the tolerance limits are higher than when it is raised rapidly. 

 For exposures of 15 to 60 minutes (Table 59), there is a wide range of lethal 

 temperatures. In general, the lethal temperatures of terrestrial animals, insects, 

 reptiles, birds, and mammals, are high, around 45° C. The lethal temperatures 

 for dwellers in moist air (earthworm, frog) are lower than for inhabitants of 

 dry air. Aquatic animals die at lower temperatures, some of those normally 

 living in cold waters dying of heat shock even below 30° C. The minimum 

 lethal temperature depends, then, on the temperature at which an animal has 

 been living and on tHe rate of change of temperature. 



ACCLIMATIZATION 



Tcmpciiiturc acclimatization occurs in nature and is of two kinds: (1) 

 genetic, which operates by selection, and (2) physiological, in which individu- 



