Resistance is futile but tolerance can explain why parasites do not always castrate their hosts.

The disease caused by parasites and pathogens often causes sublethal effects that reduce host fecundity. Theory suggests that if parasites can "target" the detrimental effects of their growth on either host mortality or fecundity, they should always fully sterilize. This is because a reduction in host fecundity does not reduce the infectious period and is therefore neutral to a horizontally transmitted infectious organism. However, in nature fully castrating parasites are relatively rare, no doubt in part because of defense mechanisms in the host. Here, we examine in detail the evolution of host defense to the sterilizing effects of parasites and show that intermediate levels of sterility tolerance are found to evolve for a wide range of cost structures. Our key result arises when the host and parasite coevolve. Investment in tolerance by the host may prevent castration, but if host defense is through resistance (by controlling the parasite's growth rate) coevolution by the parasite results in the complete loss of infected host fecundity. Resistance is therefore a waste of resources, but tolerance can explain why parasites do not castrate their hosts. Our results further emphasize the importance of tolerance as opposed to resistance to parasites.