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First published online June 12, 2009
Journal of Experimental Biology 212, vi (2009)
Copyright © 2009 The Company of Biologists Limited
doi: 10.1242/jeb.021436

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Outside JEB

THE PROBLEM OF SHIFTING THERMAL TOLERANCES

C. Jaco Klok

Arizona State University

cjklok{at}asu.edu

The interactions of organisms with the range of temperatures in their environments have generated intense interest among researchers for decades, and coupled with the wide acceptance that global climate change is a reality has resulted in a renewed interest in thermal biology. One significant aspect of ectotherm biology concerns the physiological mechanisms that designate the temperature window within which ectotherms (cold blooded animals) can maintain activity. One approach to measure the critical limits of this thermal window involves observing the temperatures where animals lose the ability to remain active by either cooling (CT_Min) or heating (CT_Max) them. This method is widely used, but recent publications suggested its outcomes are sensitive to cooling or heating rates and the temperatures at which organisms were acclimated, which could potentially complicate thermal studies in unknown ways.

Recently, in Functional Ecology, Steven Chown and colleagues reported the first systematic investigation of the effects of variable heating and cooling rates on the means and variances of CT_Min and CT_Max and the effects of acclimation at different temperatures on a model species, Drosophila melanogaster, and a globally important invasive species, Linepithema humile (Argentine ants). They acclimated the insects at 15, 20, 25 and 30°C whereafter they measured CT_Min and CT_Max at heating and cooling rates ranging from 0.05 to 0.5°C min^–1.

The results from the flies and ants were generally very different depending on the heating and cooling regimes and the temperature that the insects were acclimated to. The flies had reduced CT_Min values at slower cooling rates while acclimation at lower temperatures induced strong downward shifts in CT_Min values at all cooling rates. Measurements of critical temperatures in the ants showed that they had reduced CT_Min values at fast cooling rates while the acclimation temperature only resulted in shifts in the critical temperatures at faster heating and cooling rates. CT_Max means were similar for the two insect species with slower heating rates reducing the insects' measured CT_Max values, while the acclimation temperature shifted CT_Max values only at slower heating rates. However, the variances of CT_Max values increased slightly at faster heating rates while ants' CT_Max and CT_Min distributions about the mean showed major increases at slower heating and cooling rates.

Explanations for these incongruent results are not readily apparent yet. Nevertheless, the differences in variations about the mean induced by different cooling or heating rates can have profound consequences for estimating the extent to which these thermal tolerances can be genetically inherited by subsequent generations. If slower rates cause increases in variances, while the genetic contributions to thermal tolerance remain constant, one researcher, using slow rates, might conclude that heritability is low, while another, using faster rates, might conclude that heritability is high. This will impact on our interpretations of thermal tolerance evolvability in the light of changing environments. Likewise, the ways that variable effects of acclimation can interact with temperature rates can also lead to contradicting conclusions.

Should researchers therefore do away with these thermal methods? Chown and colleagues says that would likely not resolve the already existing issues. They advise that researchers should, rather, thoroughly document their methods and preferably use standardized rates, 0.25°C min^–1, for comparative studies, and also slower but ecologically relevant rates. And to include both standardized and ecologically relevant rates in datasets for future comparative macro-physiological studies.

References

Chown, S. L., Jumbam, K. R., Sørensen, J. G. and Terblanche, J. S. (2009). Phenotypic variance, plasticity and heritability estimates of critical thermal limits depend on methodological context. Funct. Ecol. 23,133 -140.[CrossRef]

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