Parasites figure prominently in the sexual selection literature. Whereas much attention has focused on parasitic effects on host mate choice, other areas have been sorely neglected.

Indeed, there is a dearth of studies into the role of parasites in mate competition, and into the heritability of resistance traits. As a consequence, our understanding of the genetic and evolutionary consequences of parasite-mediated sexual selection is restricted.

Recently we demonstrated that ectoparasites physically block host copulation, and in this way drive differential host mating success. We also now know that ectoparasite resistance is significantly and persistently heritable in nature. As a consequence, our work has yielded the conclusion that females are likely to be receiving indirect ("good genes") benefits as an incidental consequence (i.e., in the absence of active female choice) of this differential mating.

Some of our findings can be summarized as follows:

• Host mating effort: Hosts infected by mites adaptively up-regulate their expenditure of mating effort in relation to parasitism and probability of death.
• Host mating success: Parasitism generates variance in male mating success in the wild, and hence drives host sexual selection. Mite removal experiments confirm that this effect is caused by parasitism per se.
• Physical blocking: Parasites generate sexual selection by blocking the physical act of host copulation. The mating success of smaller males is more strongly damaged, showing that host body is a tolerance trait in this system.
• Sperm competition: In replicated tests using a red eye marker to track paternity, we found that parasitism significantly reduces sperm competitive ability (measured as P₂). Surprisingly, this effect was not dose-dependent. The physiological cause of this curious threshold effect remains unknown.