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

Abstract

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.

Manipulation of parental nutritional condition reveals competition among family members

“…In the presocial European earwig, post‐hatching parental care is not obligatory for offspring survival, as the nymphs can forage by themselves soon after hatching. Indeed, studies have shown that food restriction triggers parent–offspring competition that manifested itself in a trade‐off between the weight gains of the mothers and their offspring (Meunier & Kölliker, ; Kramer et al ., ). Furthermore, the presence of mothers can even have a negative effect on offspring survival, which means that any benefits of parental care are erased in a resource‐poor environment.…”

“…However, we have currently only limited information about the occurrence and outcome of food competition within families and in whose favour such a conflict is resolved, when all family members have the capability of feeding from the same resource. At present, there are two studies showing that under strong food limitation, the presence of a caring mother can result in a fitness loss in offspring, presumably as the female removes resources that would have been otherwise available for the offspring (Meunier & Kölliker, ; Kramer et al ., ). However, we presently do not know whether this parent–offspring competition is a common theme in facultative caring animals and what the outcome will be if both sexes provide care.…”

The other facets of family life and their role in the evolution of animal sociality

“…Intriguingly, parent–offspring competition in precocial species might even entirely negate the benefits of family living under certain harsh conditions. In line with this hypothesis, mother–offspring competition under food limitation has been shown to render maternal presence detrimental to the long‐term survival of offspring in uniparental families of the European earwig F. auricularia (Meunier & Kölliker, ; Kramer et al, ). Interestingly, highly competitive earwig mothers exhibit both a comparatively bad condition at the beginning of family life, and an increased investment into their next reproductive attempt.…”

“…However, empirical findings are sometimes at odds with these predictions. For instance, harsh conditions negate rather than increase the usual benefits of maternal presence and thus family life in the European earwig F. auricularia (Meunier & Kölliker, ; Kramer et al, ). The limited predictive power of the standard account of the evolution of family life (see Costa, ; Trumbo, ; Capodeanu‐Nägler et al, ) might partly reflect that environmental conditions, life‐history characteristics, and the benefits and costs of parental care often interact in unexpected ways (Bonsall & Klug, , b ; Meunier & Kölliker, ).…”

“…The evolution of family life shows evidence for both processes: parental care, a hallmark cooperative trait (Hamilton, ; Smiseth et al, ), greatly diversifies during the evolution of complex family systems. Conversely, the initially prevailing direct competition between parents and offspring might be progressively suppressed (Kramer et al, ). However, the evolutionary dynamics shaping family living also indicate that not all forms of cooperation might be favoured and not all conflicts equally suppressed during its consolidation.…”