Post-heatwave immune challenge reshapes selection on heatwave responses

Climate change increases the frequency and intensity of heatwaves, yet predicting selection on heatwave-induced phenotypic responses is difficult because their fitness consequences may depend on subsequent environmental conditions. We experimentally tested how variation in heatwave responses in growth, fecundity and immune activity of Lymnaea stagnalis snails covaries with long-term fitness under contrasting post-heatwave immune environments. Snails were exposed individually to a 10 day heatwave and then maintained for 60 days under low or high immune challenge (LIC or HIC). Heatwave exposure increased growth and fecundity, whereas immune traits remained unchanged, and post-heatwave immune challenge reduced reproductive output. Selection on heatwave responses was context-dependent. Snails that increased reproductive output during the heatwave achieved higher fitness, but the optimal strategy depended on the post-heatwave environment: under LIC, selection was stabilizing, favouring intermediate increases, whereas under HIC, it was positive directional, favouring the largest increases. These responses were baseline-dependent, with individuals starting from low fecundity showing the strongest increases. Growth responses experienced stabilizing selection under HIC, indicating that other performance traits can be constrained even when strong reproductive responses are favoured. These results show that subsequent ecological conditions can reshape selection on responses to extreme climatic events.