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When females care for broods of mixed paternity, there is a threat of enhanced rivalry among offspring of different paternal lineages. This competition is against the best evolutionary interest of the female because she is equally related to all of her offspring and enhanced offspring competition would decrease her fitness. It was hypothesized earlier that mothers should therefore conceal the information about paternal origin in the recognition cues used by her offspring, for example by transfer- ring the chemical cues used by her offspring for kin recognition. Here, we used a cross- fostering experiment between two closely related and sympatric earwig species, Forficula auricularia and F. decipiens, to demonstrate that F. auricularia nymphs tended by F. decipiens females differ in their cuticular hydrocarbon profiles from F. auricularia nymphs tended by F. auricularia females. This result provides evidence for maternal transfer of cuticular hydrocarbons and shows that offspring adoption across species is possible in earwigs.

Many insects raise their offspring on organic substrates or in the soil where microorganisms are abundant. Microbes may pose a serious threat to offspring development and survival by either decomposing food resources or directly infecting the offspring. Selection to cope with these effects may favor social defenses, for example, through forms of parental care that can limit or eliminate these threats to offspring fitness. In this study, we experimentally tested if maternal egg attendance in the European earwig Forficula auricularia has a function as a social defense against mold infection of eggs by manipulating exposure of eggs to mold spores and the presence of the mother in a fully factorial design. Furthermore, we investigated the potential roles of egg grooming behavior and maternal transfer of chemicals as underlying mechanisms. As predicted, the beneficial effect of egg attendance on hatching success was significantly enhanced when eggs were exposed to the mold. Females significantly increased their egg grooming duration in response to mold exposure of her eggs, and the quantity of chemicals (identified as hydrocarbons) was maintained among attended eggs but decreased substantially among unattended eggs. Maternal transfer of chemicals was confirmed in extractions of glass beads that were mingled into attended or unattended clutches. This study shows that maternal egg attendance in the European earwig has a social defense function protecting offspring against mold infection. The maternal egg grooming behavior seems to be key for this effect, probably through both the mechanical removal of spores and the continued application of chemical substances on the egg surface.