ACCLIMATION OF POIKILOTHERMIC VERTEBRATES 

 TO LOW TEMPERATURES 



C. L. Prosser 



Temperature limits thedistributionof many poikilothermic ani- 

 mals, and knowledge of responses to temperature is important for 

 physiological ecology. Natural selection acts on the capacity for 

 change within a given genotyTpe; hence it is important to learn how 

 such an environmental variable as temperature brings about bio- 

 chemical changes in individual animals. Natural variation in respect 

 to. temperature relations can best be described in terms of the re- 

 sponses to the stresses of cold and heat: survival, reproduction, 

 various rate functions, behavior. Once this variation is described 

 for natural populations of animals, it is necessary to analyze that 

 component which is genetic and that which is environmentally in- 

 duced; this analysis is permitted by acclimation of similar animals 

 to a range of temperatures. Finally, the physiological mechanisms 

 of the variation with respect to temperature can be pursued down to 

 the molecular changes, and the sequence of events by which tem- 

 perature brings about change in genetically similar individuals can 

 be elucidated. 



COMPARISON OF HOMEOTHERMS AND POIKILOTHERMS 



The differences between a poikilotherm (temperature conform- 

 er) and a homeotherm (temperature regulator) are multiple and 

 fundamental. Birds and mammals evolved from reptiles and differ 

 from present-day reptiles in possession of a thermoregulating center 

 in the brain, in insulation, in peripheral vascular responses to ambi- 

 ent temperature (which are opposite to those of reptiles), and in type 

 of metabolic compensation. Varying degrees of homeothermy ex- 

 pressed in hibernation, estivation, nocturnal temperature drop, and 



heterothermy of tissues indicate that some animals can shift from 



homeothermy to poikilothermy and that certain peripheral tissues of 



