Influence of weight asymmetry and kinship on siblicidal and cannibalistic behaviour in earwigs

In group-living or social species intraspecific predation and cannibalism can lead to substantial decreases in inclusive fitness because related individuals may encounter one another. Therefore, selection on the ability and accuracy of kin recognition to avoid losses in inclusive fitness is expected. Competition and relatedness are two key factors for the evolution of siblicide and cannibalism. However, our knowledge of the temporal dynamics and the accuracy of kin-mediated siblicide and cannibalism is scarce and experiments have usually focused on either relatedness (kin recognition) or weight asymmetry (dominance and competition) as independent components. In an experiment we set up mixed-kinship groups of three nymphs of the European earwig, Forficula auricularia Linnaeus, a gregarious and (sub-)social species, to investigate the combined effects of relatedness and weight asymmetry on the temporal dynamics of siblicide and cannibalism. We found that the temporal dynamics were jointly influenced by relatedness and weight asymmetry among nymphs. Weight asymmetry effects on siblicide and cannibalism were stronger between unrelated individuals than between related individuals as shown by a significant relatedness∗weight asymmetry interaction on survival patterns. The opportunity to choose between related and unrelated individuals did not enhance the accuracy of kin recognition but induced the above interaction between relatedness and weight asymmetry on cannibalistic outcomes. These results show that relatedness and weight affect siblicidal and cannibalistic behaviour, and that their combined effect depends on the social context.