Tim Burton, Hanna-Kaisa Lakka, and Sigurd Einum
When the environment changes from one state to another, animals and plants must often make some sort of adjustment in order to persist in their changed environment. For example, the same species of plant when grown in either cold or warm environments can develop remarkably different leaf size and structure. For many organisms, these ‘plastic’ adjustments are a well-studied phenomenon, particularly when the environment shifts from one stable state to another. However, completely stable, predictable environments are a poor representation of what actually occurs in nature, where many variables fluctuate over different time-scales, a prime example being changes in temperature. We measured changes in the heat tolerance of water fleas Daphnia magna, a large species of zooplankton important in freshwater ecosystems, in response to different patterns of temperature variation. The measurements of heat tolerance followed a 12 day long exposure to careful manipulations of temperature where the overall mean was the same but the pattern of fluctuations over time could either be (i) non-existent/stable, (ii) completely predictable or (iii) unpredictable. We observed that the heat tolerance of water fleas was not influenced by the level of variability in temperature, nor the predictability of fluctuations in temperature, but simply adjusted in response to the mean temperature the water fleas experienced 24 hours prior to measurement. These results suggest that the continuous physiological adjustments required for such fine-scale tracking of the environment can be achieved without significant cost to the organism and underscore that the responses of animals to the mean and variance in the environment may not always occur over the same time-scale.