72 



VERTEBRATE RESPIRATION 



appears that in winter the blood contains some unknown solute 

 which lowers its freezing point from —0-8° in summer to — 1-6° 

 in winter. 



Despite the relative paucity of their regulating mechanisms, 

 poikilotherms do have some control when considered over long 

 periods. For example, animals which live under cold conditions 

 usually have a higher metabolic rate at that temperature than 

 closely related species, or individuals of the same species living 

 under warmer conditions. This capacity of living organisms for 

 modifying their metabolic rates to meet the conditions under 

 which they normally live is referred to as acclimation. Fig. 20a 

 shows the relationship between metabolic rate and temperature 

 in a species which does not show any acclimation; the curves for 



(a) 



(b) 



Temp. 



Temp. 



Fig. 20. 



Metabolic rate of poikilotherms at different temperatures, (a) 

 without any acclimation, (b) showing acclimation. The curves for 

 individuals from warm conditions are shown by full lines (w) and the 

 dashed lines are for those living in cold environments (c). The dotted 

 line indicates the relationship for an animal subjected to changes that 

 are sufficiently slow for it to be acclimated at all temperatures (After 

 Bullock, T. H.: Biol Revs. Vol. 30, 1955.) 



individuals normally living at higher and lower temperatures are 

 continuous with one another. In most instances, however, the 

 two curves would be different (fig. 20b) ; the curves for tropical 

 forms being below those for arctic fishes at the temperature range 

 0-10° C. At the normal temperature of their habitat (0°), arctic 

 aquatic forms have metabolic rates which are 3 to 4 times less 



