Climate change affects organisms worldwide with profound ecological and evolutionary consequences, often increasing population extinction risk. Climatic factors can increase the strength, variability or direction of natural selection on phenotypic traits, potentially driving adaptive evolution. Phenotypic plasticity in relation to temperature can allow organisms to maintain fitness in response to increasing temperatures, thereby “buying time” for subsequent genetic adaptation and promoting evolutionary rescue. Although many studies have shown that organisms respond plastically to increasing temperatures, it is unclear if such thermal plasticity is adaptive. Moreover, we know little about how natural and sexual selection operate on thermal reaction norms reflecting such plasticity. Here, we investigate how natural and sexual selection shape phenotypic plasticity in two congeneric and phenotypically similar sympatric insect species. We show that the thermal optima for longevity and mating success differ, suggesting temperature-dependent trade-offs between survival and reproduction in both sexes. Males in these species have similar thermal reaction norm slopes but have diverged in baseline body temperature (intercepts), being higher for the more northern species. Natural selection favoured reduced thermal reaction norm slopes at high ambient temperatures, suggesting that the current level of thermal plasticity is maladaptive in the context of anthropogenic climate change and that selection now promotes thermal canalization and robustness.  Our results show that ectothermic animals also at high latitudes can suffer from overheating and challenge the common view of phenotypic plasticity as being beneficial in harsh and novel environments.

Data was collected in natural field populations at a sympatric site containing both Calopteryx splendens and C. virgo in southern Sweden: Klingavälsån at Sövdemölla Gård in Region Skåne (Latitude 55.601492, Longitude 13.657340 ) during the summers 2013-2016 (mainly June and July). Daily field work consisted of capturing and marking males and females individually, as well as re-capturing marked individuals and recording longevity and mating succcess.

We also measured microenvironmental variaton using thermal loggers (iButtons) and quantified individual variation in thermal plasticity (heating rates) using thermal imaging (infrared camera). Demographic data on survivorship and mating success were subsequently used to estimate natural and sexual selection on thermal plasticity traits (slopes and intercepts).

We provide all raw data as well as R-code needed to replicate all the statistical analyses, main figures, supporting figures and supplemental statistical tables in this paper.

The R-scripts are annotated, so that it will be possible to re-generate all the statistical analyses and all figures using the code we provide.