Parasites, immunology of hosts, and host sexual selection.

Parasite-mediated sexual selection is reviewed with special emphasis on the bird literature. Choosy females may benefit from choosing parasite-free mates if such males provide better parental care, do not transmit contagious parasites, or provide resistance genes to offspring. There is evidence in support of each of these mechanisms. The immunocompetence handicap hypothesis posits that secondary sexual characters reliably reveal the ability of males to resist parasites due to the immunosuppressive effects of testosterone and other biochemicals. Several aspects of these negative feedback mechanisms are supported by laboratory studies, but evidence from free-living animals is almost completely absent. Corticosterone rather than testosterone may potentially mediate the immunocompetence handicap mechanism. A simple version of the immunocompetence handicap is developed suggesting that body condition of male hosts is a sufficient mediator of the handicap mechanism of reliable sexual signaling. Sexual selection appears to be more intense in sexually dichromatic bird species, and comparative studies using pairwise comparisons of closely related taxa reveal that sexually dichromatic bird species have larger spleens, larger bursa of Fabricius, and higher concentrations of leukocytes than monochromatic species. Parasite-mediated sexual selection is proposed to affect parasite biology by increasing (1) the variance-to-mean ratio in parasite abundance, (2) variance in the intensity of natural selection affecting hosts, and (3) speciation rates among parasites exploiting hosts subject to intense sexual selection as compared to those subject to less intense selection.