Heat tolerance and its evolutionary potential along a latitudinal gradient in Daphnia magna

Background: Extreme weather events follow latitudinal patterns and are increasing due to climate change. To persist, natural populations must adapt to the increased occurrence of heat waves. Hypothesis: Heat tolerance is higher at lower latitudes and in smaller animals. Animals show evolutionary potential for heat tolerance. Organism: The water flea, Daphnia magna, a common species in bodies of fresh water. Methods: Set up a common-garden experiment using six populations spread across three latitudes in Western Europe, thereby creating a latitudinal gradient in local temperatures. Measure CTmax, the temperature above which animals lose motor function. Results: Thermal tolerance increased with increasing local temperatures. Body size, which was smaller at the southern latitude, negatively affected heat tolerance. Heat tolerance showed genetic variation within latitudes. Conclusions: Daphnia magna possesses the evolutionary potential to increase its heat tolerance if temperatures warm. Its heat tolerance shows latitudinal adaptation to local temperatures. A parallel pattern in body size magnifies the latitudinal pattern in heat tolerance.