Compensation for Temperature in the Metabolism and Activity of Poikilotherms

SUMMARY Many poikilothermal animals exhibit in their metabolism or activity some degree of independence of their temperature. In the general case this is regarded as reflecting a compensation rather than a fundamental insensitivity of metabolism or the rate functions measured. Illustrative cases are assembled including the following: (1) latitudinally separated populations of the same species and of different species which show adaptation in some rate function, often far from complete but sometimes apparently complete, those from higher latitudes having higher rates at given temperatures. (2) Micro‐geographic adaptation has been discovered in intertidal molluscs, comparing low‐tide level with mid‐tide level individuals of the same species; heart rates at a given temperature are higher in those from the cooler habitat. (3) Seasonal shift of various rate processes in a homoeostatic direction is likewise documented, as is (4) experimental acclimation, regarded as evidence of normal, shorter term regulations accompanying periods of unseasonable weather. (5) A number of cases show the phenomenon of rapid compensation, completed in hours or minutes and often sensibly perfect. Even when not perfect, many organisms show in one way or another that they do not submit passively to different temperatures. A number of exceptions are listed which show that the ability to compensate is far from universal. However, the same species may sometimes compensate in other rate functions. The mechanism and properties of compensation are discussed. It is concluded that even in the same organism there are several simultaneous mechanisms with different time courses and at different levels, involving enzymes, cells, organs and behaviour. The activity of several enzymes alters, as tested in homogenates from acclimated animals, but other enzymes show no change. The limiting factor in the use of temperature‐compensating mechanisms for overcoming temperature barriers to distribution is presumed to be such failure to balance the regulation of different processes. It is considered likely that at least in certain cases regulation may depend on specialized receptors for temperature. It is shown that non‐adapting, absolute temperature sense organs, such as are necessary, occur in various groups of poikilo‐therms, though it cannot be said that they function in acclimation. Acclimation is manifested by a change in the acutely* measured rate‐temperature ( R‐T ) curve in any of several ways. Commonly there is something more than a shift of the curve along the temperature axis; some upward shift as well, or a flattening of the slope, accompanies cold adaptation. The facts at hand do not permit a simple statement as to the relative importance among eurythermal species of genetically determined adaptability within a genotype and of temperature races. Both clearly exist. Some species are at first hyper‐responsive to temperature change, settling down to a steady rate after some hours, others are initially under‐responsive. Neglect of this short‐term time factor vitiates many comparisons between R‐T curves taken during dissimilar phases of response to changes. The wide distribution and demonstrated natural occurrence of acclimation and related rate compensations for temperature bespeak a large‐scale role in ecology and evolution. It is a pleasure to acknowledge the inspiration and stimulus as well as much critical thinking due to my collaborators, in particular Drs John L. Roberts, K. Pampapathi Rao, Paul A. Dehnel and Earl Segal, who themselves have contributed the original work from this laboratory.