Coping with temperature variation is part of survival in the wild for an insect whose body tmperature closely tracks the ambient environmental temperature. Yet when faced with temperature changes, such as seasonal or daily cycles, animals use a variety of tricks to compensate for these potentially lethal changes. For example, insects can alter their behavioural patterns to thermoregulate more efficiently or they might alter their physiology rapidly within their lifetime to better withstand the impending cold front. But adapting quickly to improve performance in one set of conditions, such as low temperature, might come at the cost of performing poorly at high temperatures. So a hotly debated question in insect physiology is: `do insects that are cold acclimated perform better than non-acclimated insects under cold conditions in the wild and, if so, does their improved performance come with costs if temperatures rise?'
Torsten Kristensen and colleagues at the University of Aarhus, Denmark,teamed up with Ary Hoffmann and Rebecca Hallas at the University of Melbourne in Australia to tackle this question. Using tens of thousands of Drosophila melanogaster fruit flies, the team acclimated the insects to warm or cool conditions, before releasing the insects into cool, warm or hot environments to see how they faired relative to control (non-acclimated)flies. Having dusted the conditioned flies with a coloured fluorescent powder to identify them, the insects were re-captured within a couple of days at banana-baited traps. This essentially provided a proximate measure of field fitness, since flies that are unable to reach food in the wild are far more likely to die than those that can find food. Roughly 100% of the cold-acclimated flies survived and were recaptured at 12°C relative to controls; so flies that had been given the opportunity to experience low temperatures previously were much more likely to acquire food under low temperature conditions in the wild. However, less than 0.1% of the cold-acclimated flies were recaptured at 29°C, showing that flies exposed to low temperatures previously were highly unlikely to obtain food at higher release temperatures in nature. These results clearly show that cold-acclimated flies perform better in the wild under cold temperatures, but that this improvement at low temperatures comes at a cost to high temperature performance.
However, when heat and cold tolerance of the acclimated and control flies were compared in the laboratory, the outcome differed fundamentally. As expected, cold-acclimated flies survived icy conditions better than control flies. For example, roughly 50% of the population of cold-treated flies survived –8°C whereas almost no non-acclimated flies survived these conditions. However, when the team tested the cold-acclimated flies' survival rate at high temperatures in the lab, they were in for a surprise. The cold-acclimated flies survived as well, if not better, than non-acclimated flies at 38°C. There was no apparent cost for cold acclimation at high temperatures in the lab. This result contrasts strongly with the field releases in which few of the cold-acclimated flies made it back to the banana-baited traps at higher temperatures. This finding highlights the fact that laboratory results often do not accurately predict the outcome of physiological adjustments made in the wild.
This exciting study, which hides a massive body of work in its simple elegance, has trenchant results. First, cold acclimation is indeed beneficial in the field under low temperature conditions but this comes at massive costs when flies are tested at warm temperatures. Second, these same costs and benefits were not visible when flies were assayed in the laboratory for hot and cold survival. In conclusion, this research convincingly demonstrates that the ability of flies to acclimate to changing weather is likely to be adaptive, although the extent of the response will be dependent on the relative costs and benefits of adjusting physiologically in a particular environment.