Abstract
Sexually transmitted infections (STIs) are found throughout the plant and animal kingdoms and are predicted to be key drivers of host mate choice. Similarly, changes in host mating patterns will have consequences for STI epidemiology and evolution, and so it is crucial to study hosts and STIs in the context of antagonistic coevolution. However, our understanding of host-STI coevolution is extremely limited, with few theoretical predictions for how STIs are likely to affect the evolution of host mate choice, and vice versa. Here, I present a general model of host-STI coevolution, whereby hosts can evolve a preference for healthy mates and STIs can evolve their degree of virulence. The model differs from previous work in a number of important ways, with: (1) ephemeral sexual contacts as opposed to serial monogamy; (2) both mortality and sterility virulence; (3) recovery from infection; and (4) comparisons between linear and non-linear mate choice functions. I show that coevolutionary cycling and intermediate equilibria still occur in the more general framework, but also that evolutionary branching in host mate choice is possible when mate choice is based on mortality virulence and incurs a relatively small cost. Together these findings generalise and extend our theoretical understanding of host-STI coevolution, providing increased support for parasite-mediated sexual selection as an important driver of host mate choice, and mate choice as a constraint on STI virulence.