Sexual dimorphism in immunocompetence: what does life-history theory predict?

Sexual dimorphism in immunocompetence, usually in the direction of inferior male immunocompetence, has historically been explained as the result of proximate physiological mechanisms such as the immunosuppressive effects of the male hormone testosterone. More recently, it has been argued that this pattern is best understood as a result of resource-based trade-offs between male mating effort and immune defense, a trade-off that females do not make. The central prediction of this hypothesis is that as the strength of sexual selection on males increases, the magnitude of the sex differences in immunocompetence will increase. Two implicit assumptions of this argument are that 1) longevity is of more importance for female than for male fitness and 2) that the primary benefit of immunocompetence is increased longevity. However, both of these assumptions may not be as broadly applicable as has been argued. We have modeled the optimal allocation to immunocompetence for males and females without making these assumptions. We find that the optimal allocation to immune defense for males decreases as the strength of sexual selection increases, as predicted. However, males may still invest more, relative to females, into immunocompetence if the impact of parasites on condition differs for the sexes and/or if the relationship between condition and reproduction differs for the sexes. We argue that these previously overlooked assumptions may be critical for predicting sex-specific patterns of immunocompetence.