When earwig mothers do not care to share: Parent-offspring competition and the evolution of family life

1. 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. 2. 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. 3. 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. 4. 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.