Heat waves during egg development alter maternal care and offspring quality in the European earwig

Climate change can disrupt animal fitness by reducing survival, fertility, fecundity and altering offspring development and survival. While parental care typically helps offspring cope with harsh environmental conditions, little is known about its role in buffering extreme temperature changes, such as heat waves. In this study, we tested whether parental care mitigates the impact of cold and heat waves on eggs and juveniles in the European earwig. In this insect, mothers provide obligatory egg care for about 50 days during winter, typically at temperatures around 10 °C. We exposed mothers and their eggs to three-day thermal waves of 3 °C, 10 °C (control), 17 °C or 24 °C, both 15 and 30 days after oviposition. We then measured four maternal care behaviors, maternal weight variation, as well as eggs' developmental time, survival, and hatching rate. In the resulting juveniles, we measured weight, developmental time, thermal resistance, and the expression of six heat stress and immunity genes. We found that thermal waves reduced maternal care and induced maternal weight gain. High temperatures also decreased egg hatching success, accelerated egg and nymph development, reduced the upper thermal limit of juveniles and decreased the expression of a heat shock protein (Hsp68), while other traits remained unaffected. Overall, this study highlights that access to maternal care is not enough to alleviate the stress of exposure to non-optimal temperatures during egg development in the European earwig. It also suggests that species with maternal care do not necessarily have access to effective thermal protection and may not be better adapted to climate change.