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.

Theoretical models of host^parasite coevolution assume a partially genetic basis to the variability in susceptibility to parasites among hosts, for instance as a result of genetic variation in immune function. However, few empirical data exist for free-living vertebrate hosts to support this presumption. In a crossfostering experiment with nestling great tits, by comparing nestlings of the same origin we investigated (i) the variance in host resistance against an ectoparasite due to a common genetic origin, (ii) the e¡ect of ectoparasite infestation on cell-mediated immunity and (iii) the variance in cell-mediated immunity due to a common genetic origin. Ectoparasitic hen £eas can impair the growth of nestling great tits and nestling growth was therefore taken as a measure of host susceptibility. A common origin did not account for a signi¢cant part of the variation in host susceptibility to £eas. There was no signi¢cant overall e¡ect of £eas on nestling growth or cell-mediated immunity, as assessed by a cutaneous hypersensitivity response. A common rearing environment explained a signi¢cant part of the variation in cell-mediated immunity among nestlings, mainly through its e¡ect on nestling body mass. The variation in cellmediated immunity was also related to a common origin. However, the origin-related variation in body mass did not account for the origin-related di¡erences in cell-mediated immunity. The results of the present study thus suggest heritable variation in cell-mediated immunity among nestling great tits.

Whether to reproduce once or multiple times (semelparity vs. iteroparity) is a major life-history decision that organisms have to take. Mode of parity is usually considered a species characteristic. However, recent models suggested that population properties or condition-dependent fitness payoffs could help to maintain both life-history tactics within populations. In arthropods, semelparity was also hypothesised to be a critical preadaptation for the evolution of maternal care, semelparous females being predicted to provide more care due to the absence of costs on future reproduction. The aim of this study was to characterize potential fitness payoffs and levels of maternal care in semel-and iteroparous females of the European earwig Forficula auricularia. Based on 15 traits measured in 494 females and their nymphs, our results revealed that iteroparous females laid their first clutch earlier, had more eggs in their first clutch, gained more weight during the 2 weeks following hatching of the first clutch, but produced eggs that developed more slowly than semelparous females. Among iteroparous females, the sizes of first and second clutches were significantly and positively correlated, indicating no investment trade-off between reproductive events. Iteroparous females also provided more food than semelparous ones, a result contrasting with predictions that iteroparity is incompatible with the evolution of maternal care. Finally, a controlled breeding experiment reported full mating compatibility among offspring from females of the two modes of parity, confirming that both types of females belong to one single species. Overall, these results indicate that alternative modes of parity represent coexisting life-history tactics that are likely to be condition-dependent and associated with offspring development and specific levels of maternal care in earwigs.

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The evolution of parent-offspring interactions for the provisioning of care is usually explained as the phenotypic outcome of resolved conflicting selection pressures. However, parental care and offspring solicitation are expected to have complex patterns of inheritance. Here we present a quantitative genetic model of parentoffspring interactions that allows us to investigate the evolutionary maintenance of a state of resolved conflict. We show that offspring solicitation and parental provisioning are expected to become genetically correlated through coadaptation and that their genetic architecture is dictated by an interaction between patterns of selection and the proximate mechanisms regulating supply and demand. When selection is predominately on offspring solicitation, our model suggests that the genetic correlations between provisioning and solicitation are usually positive if provisioning reduces solicitation. Conversely, when selection is predominately on parental provisioning, the correlations are mostly negative as long as parents show a positive response to offspring demand. Empirical estimates of the genetic architecture of traits involved in family interactions fit these predictions. Our model demonstrates how the evolutionary maintenance of parent-offspring interactions can result in variable patterns of coadaptation, and it provides an explanation for the diversity of family interactions within and among species.

As a consequence of the deleterious e¡ects of parasites on host ¢tness, hosts have evolved responses to minimize the negative impact of parasite infection. Facultative parasite-induced responses are favoured when the risk of infection is unpredictable and host responses are costly. In vertebrates, induced responses are generally viewed as being adaptive, although evidence for ¢tness bene¢ts arising from these responses in natural host populations is lacking. Here we provide experimental evidence for direct reproductive bene¢ts in £ea-infested great tit nests arising from exposure during egg production to £eas. In the experiment we exposed a group of birds to £eas during egg laying (the exposed group), thereby allowing for induced responses, and kept another group free of parasites (the unexposed group) over the same time period. At the start of incubation, we killed the parasites in both groups and all nests were reinfested with £eas. If induced responses occur and are adaptive, we expect that birds of the exposed group mount earlier responses and achieve higher current reproductive success than birds in the unexposed group. In agreement with this prediction, our results show that birds with nests infested during egg laying have (i) fewer breeding failures and raise a higher proportion of hatchlings to £edging age; (ii) o¡spring that reach greater body mass, grow longer feathers, and £edge earlier, and (iii) a higher number of recruits and ¢rst-year grandchildren than unexposed birds. Flea reproduction and survival did not di¡er signi¢cantly between the two treatments. These results provide the ¢rst evidence for the occurrence and the adaptiveness of induced responses against a common ectoparasite in a wild population of vertebrates.

Parental care increases the fitness of offspring at a cost to the parents in terms of residual reproductive success. This trade‐off may be affected by ecology, life history and the social environment, which raises the question as to how these factors contribute to the evolution of parental care. Here, previous hypotheses concerning the evolution of parental care in insects are summarized and discussed and the underlying empirical evidence is reviewed. Ecological factors such as harsh environments, ephemeral food sources or predation pressure are broadly accepted as evolutionary drivers of parental care. The most consistent evidence supports a role for natural enemies such as predators, microbes and cannibalistic conspecifics. Also, the importance of ecological factors may interact with the life history (parity) of a species, either as a pre‐adaptation facilitating the evolution of parental care or as a consequence of enhanced parental investment under parental care. Yet, only limited experimental research has been carried out to test the combined influence of ecology and life history in the evolution of parental care. Several forms of care can mediate the transition from solitary to family living, which entails the emergence of a novel – social – environment that generates new selection pressures from interactions within and between families. In this context, we review examples of studies on communal breeding, brood parasitism, parent–offspring conflict and co‐adaptation, and discuss how these social interactions may in turn be influenced by ecological factors such as food availability or population density. Insects are uniquely suitable for experimental and comparative research on the complex interplay between ecology, life history, and the social environment.

Evolutionary theory of parent-offspring conflict explains begging displays of nestling birds as selfish attempts to influence parental food allocation. Models predict that this conflict may be resolved by honest signaling of offspring need to parents, or by competition among nestmates, leading to escalated begging scrambles. Although the former type of models has been qualitatively supported by experimental studies, the potential for a begging component driven by scramble competition cannot be excluded by the evidence. In a brood-size manipulation experiment with great tits, Parus major, we explored the scramble component in the begging activity of great tit nestlings by investigating the mechanisms of sibling competition in relation to brood size. While under full parental compensation, the feeding rate per nestling will remain constant over all brood sizes for both types of models; the scramble begging models alone predict an increase in begging intensity with brood size, if begging costs do not arise exclusively through predation. Great tit parents adjusted feeding rates to brood size and fed nestlings at similar rates and with similar prey sizes in all three brood-size categories. Despite full parental compensation, the begging and food solicitation activities increased with experimental brood size, whereas nestling body condition deteriorated. These findings support a scramble component in begging and suggest that the competition-induced costs of food solicitation behavior play an important role in the evolution of parent-offspring communication.