Data from: 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 relatives, and hence is often regarded as central process in social evolution. Surprisingly, however, our understanding of the role of kin competition in the evolution of family life remains fragmentary, despite the close scrutiny it received in studies on sibling rivarly. This is because much less attention has been given to local competition between parents and their offspring, and its potential impact on the early evolution of parental care and family life. 2. Here, we examined mother-offspring competition over food access in the European earwig Forficula auricularia, an insect with facultative family life. Specifically, we (i) raised earwig offspring under food limitation either together with or without their mother, and then (ii) tested whether and how the – potentially competitive – weight gains of mothers and offspring during family life affected the offsprings’ survival rate and morphology, or the future reproductive investment of their mother. 3. In line with a mother-offspring competition over food access, we showed that high maternal weight gains during family life reduced the survival prospects of maternally tended offspring, while they increased maternal investment into second clutch production (but not the body size of the surviving offspring). Conversely, high offspring weight gains generally increased the offsprings’ survival, particularly 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 local competition between parents and their offspring can counteract the benefits of (facultative) parental care, and may thus impede the evolution of family life in resource-poor environments.1. 亲缘竞争通常会削弱亲属间合作的收益，有时甚至会完全抵消这些收益，因此常被视为社会演化的核心过程。然而令人意外的是，尽管亲缘竞争在同胞竞争相关研究中已得到深入探讨，但我们对其在家庭生活演化中所扮演的角色的认知仍较为碎片化。这是因为学界对亲代与子代间的局部竞争及其对亲代抚育和家庭生活早期演化的潜在影响关注甚少。 2. 本研究以兼性家庭生活的昆虫欧洲球螋（Forficula auricularia）为对象，探究了其亲代与子代间的食物获取竞争。具体而言，我们（i）将球螋若虫置于食物限制条件下，分别与母虫共同饲养或单独饲养；（ii）随后检验家庭生活期间母虫与若虫的（潜在竞争性）体重增益，是否以及如何影响若虫的存活率与体型形态，或是母虫的未来繁殖投入。 3. 与亲代-子代食物竞争的预期一致，我们发现家庭生活期间母虫的高体重增益会降低受其抚育的若虫的存活概率，但会提升母虫对第二窝繁殖的投入（但不影响存活若虫的体型大小）。与之相反，若虫的高体重增益通常会提升自身的存活率，尤其是在与母虫共同饲养的情况下。值得注意的是，初始体重较低的母虫，其体重增益尤为显著。 4. 综上，我们的研究结果表明，在食物限制条件下，母虫的存在会引发亲代与子代间的局部竞争。这种竞争限制了若虫的存活，但使得母虫能够提升其未来繁殖投入，或维持当前的身体状态。从更宏观的层面来看，我们的研究发现亲代与子代间的局部竞争可以抵消（兼性）亲代抚育的收益，因此可能会阻碍资源匮乏环境中家庭生活的演化。