PROSSER 



When environmental temperature drops, the temperature of a 

 poikilotherm (e. g., fish) drops with it. Any metabolizing organism 

 produces some heat, and the liver of a large fish may be significantly 

 warmer than its environment. But poikilotherms lack insulation, and 

 their body temperatures are virtually the same as that of their en- 

 vironments. If the drop in temperature is rapid and considerable, the 

 poikilotherm may enter a chill coma and even die from respiratory 

 failure. If the cold stress is less, there may be initial stimulation, 

 increased nervous activity (well shown in crustaceans and insects) 

 and an initial transient increase in oxygen consumption, the so-called 

 initial shock reaction. This is followed by a decline of metabolism to 

 a stabilized state which corresponds to the reduced temperature. The 

 Q for metabolism is usually between 2.0 and 2.5; hence the meta- 

 bolic response to temperature is steeper than the change in body 

 temperature. With time (days or weeks) some metabolic compensa- 

 tion may occur. The compensatory changes for either a fall or a rise 

 in temperature in a poikilotherm are diagrammed in Figure 2. The 

 time course of acclimation differs according to the function mea- 

 sured and the kind of animal. 



Precht (Precht, 1958;Prechtetal., 19 55) has classified the pat- 

 terns of acclimation as indicated in Figure 3 and has termed them 

 capacity adaptations. The five possible patterns are: (1) overcom- 

 pensation so that metabolism is higher in the cold than at the initial 

 temperature, (2) perfect compensation with the same metabolism at 

 each temperature, (3) partial compensation, (4) no compensation, the 

 metabolism continuing to follow the van't Hoff relation, and (5) in- 

 verse compensation or further reduction in metabolism. The com- 

 monest pattern of acclimation is the third, partial compensation, so 

 that if the metabolism of animals from temperatures t and t is 

 measured at the same intermediate temperature, the one acclimated 

 to the cold has a higher metabolism. This acclimation pattern can 

 apply to other rate functions besides metabolism- heart rate, breath- 

 ing rate etc. A comparable sequence is described for moderate in- 

 creases in temperature (Precht, 1958). Acclimation to heat is a re- 

 duction in metabolism below the initial level determined by the Q 



10 

 relation (Gelineo, 1959). The net effect of long-term acclimation is 



to tend toward relative constancy of energy liberation despite 

 changes in body temperature. 



