Plasticity in response to mean temperature and temperature variation - from life history to heat tolerance and...

Climate warming has many direct and downstream effects on animals. For example, warmer developmental temperatures can reduce insect melanism, which is related to thermoregulation, immunity, desiccation resistance and life history. Increased temperature variation is also a feature of climate change, and it may have a larger impact on animals than warming. Here, we examined the combined effects of mean temperature and temperature variation on life history, heat tolerance and melanism. We determined thermal plasticity using a factorial manipulation of mean temperature (20, 25 and 30°C) and daily temperature fluctuation (±0, 5 and 10°C) during development in the variable field cricket (Gryllus lineaticeps). We tested hypotheses comparing thermal plasticity due to (1) mean temperature versus (2) temperature variation, (3) the interdependency of mean temperature and temperature variation in thermal plasticity (i.e. interactive effects on traits), and (4) whether life-history strategy (i.e. investment in dispersal versus reproduction) influences thermal plasticity. Mean temperature had stronger effects on daily accumulated heat and on traits than temperature variation, yet interactive effects were common, and their effect sizes were stronger than mean temperature alone for body mass and size, and reproductive investment. Warmer, more thermally variable environments of the future may be particularly costly. Flight-capable individuals differed in their responses to mean temperature and/or temperature variation regarding developmental rate, body size and mass, reproductive investment and melanism. In sum, combined shifts in mean temperature and temperature variation strongly influence life-history strategy, heat tolerance and coloration, all of which may be critical to animals' resilience in the face of climate change.