Altitudinal life-history variation and thermal adaptation in the copper butterfly Lycaena tityrus

Understanding how organisms adapt to complex environments lies at the very heart of ecology and evolutionary biology. Clinal variation in traits related to fitness suggests a contribution of directional selection, and analyzing such variation has consequently become a key element in investigating adaptive evolution. In this study we examine climatic adaptation in the temperate-zone butterfly Lycaena tityrus across replicated populations from low-, (mid-) and high-altitudes, each reared at two different temperatures. In common garden experiments, high- compared to low-altitude populations showed a longer development time accompanied by reduced larval growth rates, increased cold- but decreased heat-stress resistance, and increased flight duration across a range of ambient temperatures. In contrast, differences in morphological traits such as pupal mass or wing size were negligible, suggesting that morphology is not necessarily indicative of flight performance. While patterns in stress resistance traits suggest adaptation to local temperatures, development times between populations were associated with differences in season length (enabling a second generation at lower altitudes, while high-altitude populations are monovoltine) rather than with temperature per se. Mid-altitude populations showed either intermediate patterns or patterns resembling low-altitude populations. Plastic responses to different rearing temperatures resulted, as expected, in reduced larval and pupal development times at higher temperatures accompanied by higher growth rates and decreased pupal mass. Further, butterflies reared at a lower temperature showed reduced chill-coma recovery times and decreased heat knock-down resistance as compared to those reared at a higher temperature. In summary, this study demonstrates local adaptations to regional climates, and that environmentally-induced plasticity can be as important as genetic factors in mediating adaptive responses.