Abstract
Proper assessment of environmental resistance of animals is critical for the ability of researchers to understand how variation in environmental conditions influence population and species abundance. This is also the case for studies of upper thermal limits in insects, where researchers studying animals under laboratory conditions must select appropriate methodology on which conclusions can be drawn. Ideally these methods should precisely estimate the trait of interest and also be biological meaningful. In an attempt to develop such tests it has been proposed that thermal ramping assays are useful assays for small insects because they incorporate an ecologically relevant gradual temperature change. However, recent model-based papers have suggested that estimates of thermal resistance may be strongly confounded by simultaneous starvation and dehydration stress. In the present study we empirically test these model predictions using two sets of independent experiments. We clearly demonstrate that results from ramping assays of small insects (Drosophila melanogaster) are not compromised by starvation- or dehydration-stress. Firstly we show that the mild disturbance of water and energy balance of D. melanogaster experienced during the ramping tests does not confound heat tolerance estimates. Secondly we show that flies pre-exposed to starvation and dehydration have “normal” heat tolerance and that resistance to heat stress is independent of the energetic and water status of the flies. On the basis of our results we discuss the assumptions used in recent model papers and present arguments as to why the ramping assay is both a valid and ecologically relevant way to measure thermal resistance in insects.
Highlights
Researchers investigating thermal tolerance in ectothermic animals are often challenged when designing appropriate laboratory assays
Time impacts on thermal tolerance measures because loss of function or mortality may be induced by either short and extreme temperatures or through longer and more moderate temperature exposures [1,2,3,4,5,6]. Given this interaction it becomes difficult to define a universal and ‘‘true’’ thermal limit. This problem is further accentuated by the fact that animals display marked hardening and acclimation responses where even very short preexposures to heat or cold may alter the inherent thermal tolerance [7,8,9]
Low environmental humidity is an important stressor for small insects and high temperature severely limits desiccation tolerance our results show that moderate desiccation and starvation does not limit heat tolerance per se during short term stress exposures experienced during ramping assays
Summary
Researchers investigating thermal tolerance in ectothermic animals are often challenged when designing appropriate laboratory assays. The most important argument for using ramping assays is that gradual exposures are more ‘‘ecologically relevant’’, i.e. animals are exposed to gradually changing temperatures in their natural environment (recently reviewed by [15]) It has been highlighted how gradual exposure to increasing or decreasing temperatures allows time for the expression of physiological coping mechanisms [4,14] and depends on the actual rate of temperature change [11,12,16]. This might not be possible in assays where animals are exposed abruptly to extreme temperatures [15]
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