Early-life thermal stress reduces survival but has no long-term effects on reproduction in an insect

Environmental temperature is a key determinant of fitness, influencing survival, reproduction, activity and behaviour. With climate change increasing the frequency and intensity of extreme thermal events, it is crucial to understand the capacity of organisms to adapt to a new thermal reality. While the impacts of acute thermal stress on physiological traits are well-documented, less is known about the ontogenetic effects of such extreme events, particularly regarding how they interact with complex behavioural mechanisms like parental care. Here, we investigated how exposure to a heatwave during the larval stage influences adult reproductive success and parental care in the burying beetle Nicrophorus vespilloides. Larvae were exposed to a simulated heatwave (26 °C for 72 h) or maintained under control conditions (20 °C). We then assessed reproductive and behavioural outcomes in adulthood, along with downstream effects on offspring fitness. We found that larvae exposed to a heatwave suffered a significant reduction in survival to adulthood. However, for individuals that survived, there was no significant effect on their reproductive success, the amount of parental care provided to their offspring, or their offspring's fitness. These findings offer a nuanced perspective on the prevailing assumption that fertility and reproductive behaviour are more sensitive to thermal stress than survival. Our results underscore the need to consider stage-specific and trait-specific responses when evaluating the biological impacts of climate extremes and highlight the complexity of thermal resilience in ectotherm life histories.