He's healthy, but will he survive the plague? Possible constraints on mate choice for disease resistance

Can females enhance their fitness by choosing a mate based on his disease resistance in addition to his current health and robustness (i.e. male condition)? The complex nature of disease resistance may constrain the evolution of female choice for this trait. Using a mathematical model, we found that choice for immune function (an element of disease resistance) provided females with a fitness advantage. However, the fitness advantage was often small, much smaller than the fitness advantage that females obtained from mating with males in good condition. Females choosing for a combination of male condition and male immune function sometimes showed no fitness advantage compared with females choosing for condition alone, even when condition and immune function were positively correlated. Our results suggest that when condition and immune function are correlated, selection for choice for male immune function may be driven by the fitness advantage that comes from mating with males in the best condition, even if a sexually selected trait correlates with male immune function. Moreover, females choosing for males with maximal immune function produced offspring with immune functions above the level needed for maximal fitness. In some species, females may gain little or no fitness advantage by choosing for male immune function per se in addition to male condition, which may explain why not all studies find evidence for female choice for male immunocompetence.