IMMUNE DEFENSE, EXTRA-PAIR PATERNITY, AND SEXUAL SELECTION IN BIRDS

Secondary sexual characters have been suggested to reliably reflect th e ability of individuals to resist debilitating parasites, and females may gain direct or indirect fitness benefits from preferring the most extravagantly ornamented males. Extra-pair paternity provides an esti mate of an important component of sexual selection in birds. Species w ith a high frequency of extra-pair paternity have a variance in realiz ed reproductive success that is greater than the variance in apparent reproductive success, and extrapair copulations and hence extra-pair p aternity by females are often directly associated with the expression of male secondary sexual characters. If sexually dichromatic species h ave experienced a long period of antagonistic coevolution with their p arasites, such species should have evolved larger immune defence organ s than sexually monochromatic species. Bird species with sexual dichro matism had larger spleens for their body size than monochromatic speci es in a comparative analysis. Furthermore, species with a high frequen cy of extra-pair paternity were sexually dichromatic and had large spl eens for their body size. These results are consistent with the hypoth esis that females of dichromatic bird species seek extra-pair copulati ons to obtain indirect fitness benefits in terms of superior resistanc e of their offspring to virulent parasites.