Age, sex, mating status, but not social isolation interact to shape basal immunity in a group-living insect.

Immunity is a crucial but costly trait. Individuals should therefore adjust their investment into immunity to their condition and infection risks, which are often determined by their age, sex, mating status and social environment. However, whether and how these four key factors can interact to shape basal immunity remains poorly understood. Here, we tested the simultaneous effects of these factors on hemocyte concentration and phenoloxidase activity in adults of the European earwig. We found that hemocyte concentration increased with age, and that this increase was stronger in males. We also found an age-dependent increase in phenoloxidase activity in males and virgin females, but not in mated females. However, the two immune parameters were independent of social isolation. Overall, our results reveal that a complex interplay between age, sex and mating status determines basal immunity and stress the importance of interactions in our understanding of immune investment.