When solitude comes too soon: premature social isolation disrupts female reproductive performance in the European earwig

Social life is common in animals, yet individuals often experience periods of social isolation that are increasingly recognized as disrupting behaviour, physiology and development. However, whether these disruptions translate into direct fitness costs remains poorly understood, limiting our understanding of the evolutionary consequences of social isolation. Here, we addressed this gap by testing the effects of social isolation on female reproductive performance in the European earwig, Forficula auricularia . In this insect, adults are predominantly social and typically aggregate for several months, with females isolating only in early winter to lay eggs. We experimentally isolated mated females either 40 days before, 20 days before or on the day they typically begin solitary living and measured eight reproductive traits alongside female survival. To assess whether immune challenge modulates isolation costs, we also exposed females to the bacterium Serratia marcescens . We found that females isolated earliest were less likely to oviposit, laid eggs substantially later, produced smaller clutches and experienced higher rates of complete clutch failure than females isolated later. These effects were independent of pathogen exposure and female survival and occurred even though females’ spermathecae contained viable sperm before isolation. Overall, our results show that social isolation directly reduces female fitness, suggesting that the reproductive costs of early isolation may favour prolonged group living in facultatively social species such as the European earwig. More generally, these findings point to potential evolutionary feedback in which extended isolation erodes reproductive performance, thereby reinforcing the maintenance of group living. Future work is needed to elucidate the underlying mechanisms, including effects on oogenesis, egg viability and other reproductive processes.