Kin competition often reduces – and sometimes entirely negates – the benefits of cooperation among family members. Surprisingly, the impact of kin competition on the fitness effects of family life only received close scrutiny in studies on sibling rivalry, whereas the possibility of parent–offspring competition has attracted much less attention. As a consequence, it remains unclear whether and how parent–offspring competition could have affected the early evolution of parental care and family life. Here, we examined the occurrence and consequences of parent–offspring competition over food access in the European earwig Forficula auricularia, an insect with facultative family life reminiscent of an ancestral state. Specifically, we (1) raised earwig offspring under food limitation either together with or without their mother, and (2) tested whether and how the — potentially competitive — weight gains of mothers and offspring during family life affected the offsprings' survival rate and morphology, and the future reproductive investment of their mother. In line with the occurrence of parent–offspring competition, we showed that high maternal weight gains during family life reduced the survival prospects of maternally tended offspring, while they increased the mothers' investment into the production of a second clutch (but not the body size of the surviving offspring). Conversely, high offspring weight gains generally increased the offsprings' survival, but did so to a larger extent when they were together with their mother. Intriguingly, mothers that had exhibited a low initial weight showed especially high weight gains. Overall, our results demonstrate that maternal presence under food restriction triggered a local competition between mothers and their offspring. This competition limited offspring survival, but allowed mothers to increase their investment into future reproduction and/or to maintain their current body condition. On a general level, our findings reveal that parent–offspring competition can counteract the benefits of (facultative) parental care, and may thus impede the evolution of family life in resource‐poor environments. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12915/suppinfo is available for this article.

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BackgroundTo optimize their resistance against pathogen infection, individuals are expected to find the right balance between investing into the immune system and other life history traits. In vertebrates, several factors were shown to critically affect the direction of this balance, such as the developmental stage of an individual, its current risk of infection and/or its access to external help such as parental care. However, the independent and/or interactive effects of these factors on immunity remain poorly studied in insects.ResultsHere, we manipulated maternal presence and pathogen exposure in families of the European earwig Forficula auricularia to measure whether and how the survival rate and investment into two key immune parameters changed during offspring development. The pathogen was the entomopathogenic fungus Metarhizium brunneum and the immune parameters were hemocyte concentration and phenol/pro-phenoloxidase enzyme activity (total-PO). Our results surprisingly showed that maternal presence had no effect on offspring immunity, but reduced offspring survival. Pathogen exposure also lowered the survival of offspring during their early development. The concentration of hemocytes and the total-PO activity increased during development, to be eventually higher in adult females compared to adult males. Finally, pathogen exposure overall increased the concentration of hemocytes—but not the total-PO activity—in adults, while it had no effect on these measures in offspring.ConclusionsOur results show that, independent of their infection risk and developmental stage, maternal presence does not shape immune defense in young earwigs. This reveals that pathogen pressure is not a universal evolutionary driver of the emergence and maintenance of post-hatching maternal care in insects.

Social immunity offers protection against pathogens in group living species • Mostly studied in eusocial insects, it can be found in many social systems • Studying its presence in more non-eusocial systems would provide major benefits • It would help deciphering evolutionary pathways of individual and social immunity • It would help better understanding the roles of pathogens in social evolution

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