Monogamy removes constraints on reproductive tissue investment imposed by intense sexual selection.

A key factor influencing the reproductive success and fitness of an individual is the allocation of resources to reproductive tissue. Previous research has documented positive influences of sexual selection on the evolution of male traits such as testes size. However, few studies have focused on whether sexual selection shapes overall reproductive tissue investment patterns, and even fewer have studied it in both males and females. Furthermore, whether the ecological/demographic context modulates these effects is not known. We used experimental evolution on an insect model system to assess the independent and interactive effects of sexual selection intensity (intense [polygamy] vs. relaxed [monogamy]) and metapopulation structure (absent vs. present) on reproductive tissue investment in both sexes. Counterintuitively, we found that both males and females evolving under monogamy had higher relative reproductive tissue investment than individuals evolving under polygamy. We found no independent effect of metapopulation structure or any interactive effect with sexual selection on reproductive tissue investment of either sex. Our findings suggest that trade-offs between pre- and post-copulatory sexual selection may constrain reproductive tissue investment under intense sexual selection, and that selection under monogamy removes this constraint. Generally, our study underscores the complexities underpinning the evolution of reproductive allocation strategies.