Males can evolve lower resistance to sexually transmitted infections to infect their mates and thereby increase their own fitness

Sexually transmitted infections (STIs) often lower their host’s future reproductive success by inducing sterility. Females can minimise the reproductive cost of infection by plastically increasing their current reproductive effort (i.e. terminal investment) before they become sterile. In polyandrous systems, long-term female survival or fecundity is often irrelevant to male fitness. Mating with an infected, terminally investing female potentially yields greater fitness gains for males than mating with an uninfected female. Males might consequently benefit from infecting females with an STI. We construct mathematical models of the evolutionary consequences of a sterilising STI. We show that females should terminally invest in response to an STI when immune investment is relatively ineffective at delaying STI-induced sterility. Cost-effective immune responses may conversely select for reduced reproductive effort after infection (‘terminal divestment’). Crucially, we then show that female terminal investment can select for lower STI resistance in males. This selection is driven by fitness gains to males that acquire the STI and subsequently infect their mates, which offset any costs of infection (e.g. male sterility). This type of adaptive mate harm generates sexual conflict over the optimal level of resistance to STIs. It could partly explain why immune reactions to new infections are weaker in males than females of many species.