Making sense of heat tolerance estimates in ectotherms: lessons fromDrosophila

Abstract

Summary1. An increasing body of knowledge suggests that the estimation of critical upper thermal limits (CT max ) is highly dependent on the experimental methodology employed. Here, we employ a theoretical approach to analyse how estimates of CT max (knock-down temperatures and times) are affected by measurement protocol. 2. Our model is able to reproduce the results of empirical studies on Drosophila melanogaster, suggesting that it adequately mimics organismal responses during assays. With simulated data sets, we also show that many experimental protocols result in unreliable and often highly biased estimates of CT max in Drosophila and possibly in other ectotherms. 3. The confounding effects of stochasticity, resource depletion (or fatigue) and short-term acclimatory responses are expected to be higher in longer assays, and therefore, short assays should be generally preferred. The experimental protocol of choice must also take into consideration the range in which measurement accuracy is not affected and the potential problems of thermal inertia in larger organisms. 4. Our findings justify previous concerns that the methodology may have a greater impact on estimates of CT max than the biological process under study, and explain why many studies on the subject have often reported inconsistent and even contradictory results.

An invitation to measure insect cold tolerance: Methods, approaches, and workflow

“…It is clear that the rate of temperature change does affect the response of the insect (this has also been vigorously discussed in the context of high temperature tolerances, e.g. Santos et al, 2011;Terblanche et al, 2007;Terblanche et al, 2011). Thus, it is important to maintain consistent cooling rates among sites or species for comparisons, and to be cognizant of the assumptions inherent in comparing thermal limits among studies that used different exposure conditions.…”

“…Kleynhans et al, 2014;MacMillan and Sinclair, 2011b;Terblanche et al, 2006). Note that there has been considerable recent debate on the 'correct' rate for measuring high temperature tolerances Santos et al, 2011;Terblanche et al, 2007;Terblanche et al, 2011), and it is therefore imperative to clearly report the conditions when describing CT min experiments. To expedite measurements, it should be possible to rapidly reduce the temperature from the rearing temperature to an intermediate temperature (e.g.…”

Cellular damage as induced by high temperature is dependent on rate of temperature change – investigating consequences of ramping rates on molecular and organismal phenotypes in Drosophila melanogaster Meigen 1830

“…However, methodology including the choice of rate of temperature change has a profound effect on estimates of absolute values of critical thermal limits. This has sparked a debate on the underlying causes for, and interpretation of, this variation Santos et al, 2011;Terblanche et al, 2011;Overgaard et al, 2012). A better understanding of the role of methodology will probably yield a better understanding of temperature limits and field responses to environmental temperatures in arthropods (e.g.…”

“…Our key point was to describe physiological responses to different ramping rates and to provide information on the physiological background for the commonly observed divergence in measurements of thermal tolerance (CT max ) using different ramping rates. Furthermore, we provide results of general interest for the basic understanding of physiological adaptation to high temperature in ectotherms, which may also contribute to an ongoing debate about how best to measure thermal resistance in laboratory assays Santos et al, 2011;Terblanche et al, 2011;Overgaard et al, 2012). We test the following hypotheses:…”

“…A better understanding of the role of methodology will probably yield a better understanding of temperature limits and field responses to environmental temperatures in arthropods (e.g. Terblanche et al, 2007;Overgaard et al, 2011;Santos et al, 2011).…”