348 



Comparative Animal Physiology 



whereas prawns of Plymouth, England, from 15° water, survived well at 

 temperatures up to 17°.'''' Animals from cold waters have lower lethal tem- 

 peratures than those from warm waters, and for animals from a given region 

 the temperature causing heat death is higher in summer than in winter. 



Acclimatization of individuals (physiological acclimatization) also occurs. 

 Data for several species of fish reported by Hathaway^'^ are shown in Table 60. 

 The effects of acclimatization on lethal temperatures of goldfish have been 

 well studied by Fry and his associates (Fig. 86).^'^ The upper lethal tempera- 



TABLE 60. TEMPERATURE TOLERATED FOR 1 HOUR ONLY" 



ture increased 1 degree centigrade for every 3 degree rise in acclimatization 

 temperature up to 36.5° C, when the high lethal was 41° C; the lower lethal 

 temperature decreased 2 degrees for every 3 degree fall in acclimatization tem- 

 perature down to 17° C, when the low lethal temperature was 0° C. Ap- 

 parently the mechanisms causing heat or cold death are labile. Seasonal differ- 

 ences in lethal temperatures are largely due to acclimatization, but other 

 factors, such as endocrine changes, cannot be disregarded. 



Some of the numerous alterations which might account for temperature 

 acclimatization in cold-blooded animals are well discussed by Fox.^'- ^'^ Animals 

 from a cold environment show increased sensitivity to changes in temperature. 

 The rate of pulsation in the dorsal blood vessel in Perinereis and of the heart 

 in several crustaceans from Plymouth, England, is higher at a given tempera- 

 ture and increases faster with rising temperature than in animals from Tamaris 

 (Mediterranean). At a given temperature the oxygen consumption of animals 

 from cold waters is higher than the oxygen consumption of the same species 

 from warmer waters. For example, the prawn Pandalus montagui from Kris- 

 tineberg (5°-7°) had a Q02 nearly twice that of prawns from 15° water at 

 Plymouth, when both were measured at 10.5°,^" and river lampreys kept at 

 3° consumed 50 per cent more oxygen than lampreys from 16° water, when 

 both were measured at 16°. ^^^^ However, at the temperatures at which they 

 normally live, the metabolism of warm-water poikilotherms is higher than that 

 of arctic species, yet breathing rates and heart rates are not proportionately 

 higher; hence animals from warm water have more "non-activity" metabolism. 

 Individuals of a poikilothermic species reared at low temperature are usually 

 larger than individuals from warmer waters.*^- Planaria tested at 30° for sensi- 

 tivity to cyanide show greater sensitivity if reared at 20° than if reared at 30°." 

 Clones of Dapliuia with distinctly different temperature optima have been 

 segregated."' 



Non-hibernating homoiotherms can also become acclimatized to temperature 

 extremes. The initial effect of temperature extremes in mammals is stimulation 



