Heat stress disrupts maternal care through independent effects on mothers and eggs

Parental care typically protects offspring from environmental stress. Yet, it remains unclear whether this protection arises from stress effects on parents, offspring or their interactions. We addressed this gap in the European earwig (Forficula auricularia) by independently manipulating heat stress in mothers and eggs using a cross-fostering experiment and measuring maternal care and offspring development. Our data show that heat stress altered care in additive rather than interactive ways: mothers adjusted egg collection in response to their own exposure and egg grooming in response to the eggs' exposure. Mothers also influenced embryonic development after oviposition, as eggs developed faster when foster mothers had experienced heat stress. Finally, heat stress on eggs increased hatching asynchrony, reduced juvenile survival and thermal tolerance, and altered egg morphology and chemical profiles, revealing the limited buffering capacity of maternal egg care. Overall, these results reveal that parental care in earwigs responds plastically to acute thermal stress through independent pathways in mothers and eggs. These findings highlight the behavioural flexibility of family interactions under heat stress and the limits of parental care in compensating for direct thermal damage, providing insight into how families may cope with increasingly frequent heatwaves under climate change.