Decreased levels of blood trypanosome infection correlate with female expression of a male secondary sexual trait: implications for sexual selection

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Abstract
Hamilton & Zuk proposed that conspicuous male plumages of birds could be reliable signals of parasite resistance. We examined this prediction in a system involving blood trypanosomes in a passerine bird, the pied flycatcher (Ficedula hypoleuca). Trypanosome prevalence and intensity of infections were unrelated to male age and expression of a secondary sexual trait. However, trypanosome infections were absent or maintained at significantly lower levels in females expressing a male trait which females have been shown to prefer in males. This relation held even after controlling for the significant effects that female age had on both the expression of the male trait and the rate of parasitism by trypanosomes. Given that the male trait is heritable and genetically correlated between the sexes, and there exists assortative mating based on white patch size and/or expression, a system of mutual mate choice may exist. In such a system, the benefits accrued to both males and females would be a higher chance of successful mating and the acquisition of genes for resistance to parasites to transmit to the offspring. These data thus support by an alternate route Hamilton & Zuk's hypothesis on parasite-driven sexual selection.