Benefits and costs of earwig (Forficula auricularia) family life

Abstract

The evolution of parental care and family group formation critically depends on offspring survival benefits and parental fecundity costs of care under given ecological conditions. Investigations of the functional significance of care in insect species that exhibit facultative parental care have been relatively rare but may be of particular interest for better understanding of benefit and cost schedules at an early evolutionary stage. In this study, aspects of benefits and costs of care were addressed in the sub-social European earwig (Forficula auricularia; Dermaptera: Forficulidae) by manipulating the presence of tending mothers and brood size in a fully crossed experimental design. Larvae growing in broods tended by their mother or of reduced size showed a higher survival probability than larvae growing in untended or large broods, as predicted if maternal care is beneficial and shaped by a trade-off between number and quality of offspring. Analysis of patterns of food consumption and developmental time further suggested that the benefit of maternal attendance is mediated by the maternal provisioning of food, while the quality-quantity trade-off seemed to be driven by sibling rivalry. Further, tending mothers delayed the production of a second clutch, indicating a potential cost of care in terms of lifetime fecundity. This study experimentally shows benefits and potential costs of maternal care and family group formation in the European earwig. More detailed behavioural experiments will be required to fully understand how behavioural interactions among family members mediate these reproductive outcomes.

Short-term benefits, but transgenerational costs of maternal loss in an insect with facultative maternal care

“…[4,28]). Previous studies already revealed that the maternal presence reduces nymph survival when families had restricted food access [21], but increased nymph survival when families had access to low quality food [15]. Here, the effects of maternal presence under ad libitum, high quality food could reveal an increased expression of sibling rivalry when juveniles have access to maternal resources, as proposed in a recent model [29].…”

“…However, investigating the occurrence of these effects in precocial invertebrates, in which juveniles exhibit early foraging capabilities and consequently only facultatively rely on parental resources [14,15], could provide crucial information on the early evolution of parental care. Indeed, transgenerational costs could be a key promoter of the maintenance of family life when parental loss has limited (if any) short-term costs in terms of offspring survival, a scenario that applies to precocial systems and probably prevailed in the early evolution of family life [16].…”

When it is costly to have a caring mother: food limitation erases the benefits of parental care in earwigs

“…But such parental care often comes at costs to parents (see [1,2] for a review). For instance, it may reduce their capability to invest in future reproduction, delay successive broods or decrease their likelihood to survive until the next reproductive period [3,4]. To maximize fitness returns on investment, parents are expected to adjust their level of care in relation to variation in benefits to offspring and in costs to themselves [5].…”

“…In line with this idea, positive associations between resource availability and parental care have been reported in a few altricial species where offspring cannot survive without care (reviewed in [1]). While such patterns provide information about when parents should stop caring owing to enhanced costs, these studies are of limited relevance to understand how environmental quality shapes the early evolution of parental care and family life [4,11,12].…”

“…This species is ideal for our question since mothers protect their clutches against natural enemies, provide food to their young and their presence improves offspring (nymph) survival under ad libitum food conditions. Furthermore, nymphs can survive and feed for themselves in the absence of mothers, and females can lay a second and final clutch a few weeks after having tended first clutch nymphs [4,13,14]. To assess whether maternal presence possibly decreases offspring survival by exacerbating the effects of sibling competition (partly mediated by sibling cannibalism [15]), we also manipulated brood size [4] and siblings' relatedness [15] in addition to maternal presence.…”