PROSSEP 



changes which occur in homeotherms, the nervous changes under- 

 lying behavior are of particular importance in poikilotherms. 



Resistance t o Temperature Extremes 



In nature, the adaptations favoring survival at extremes of heat 



or cold may be more important than compensations in the mid- range. 



Some geographic races of fish (e. g., Salvelinus alpinus) have been 



shown to differ in their lethal temperatures but not in temperatures 



of maximum cruising speed (McCauley, 1958). The literature on 



change of lethal temperature with acclimation is extensive (Brett, 



1956), but very little is known of the responsible cellular changes. 



Some organs are more sensitive than others; for example, the brain 



is more sensitive than the heart, but the chemical bases for such 



differences are unknown. Differences in inactivation temperatures 



have been obseived for some enzymes from thermophilic and meso- 



philic bacteria (Koffler, 1957), and the inactivation temperature for 



amylases, pepsin, and trypsin from fish is lower than for the same 



enzymes from mammals (Chesley, 1934; Vonk, 1941). Acetylcholine 



acetylase of ^ fish (Labrus) brain is maximally active at 25 C and 



c 



is inactivated at 37 G, whereas corresponding temperatures for the 







same enzyme from rabbit brain are 42 C and 47 C respectively 

 (Milton, 1958). Changes in lipids as shown by their melting points 

 may be important for cell permeability. The effect of endocrines, 

 such as the thyroid, on heat death was mentioned above. It may well 

 be that more marked changes occur in resistance to temperature 

 extremes than as metabolic compensations within the normal range, 

 and there may be little relation between the compensations of capa- 

 city adaptation and the stress responses of resistance adaptation. 



CONCLUSIONS 



Acclimation of poikilothermic vertebrates to temperature is 

 basically different from that in homeotherms in that compensations 

 of poikilotherms tend toward maintenance of relatively constant 



26 



