PARASITES, IMMUNOLOGY OF HOSTS, AND HOST SEXUAL SELECTION

Parasite-mediated sexual selection is reviewed with special emphasis o n the bird literature. Choosy females may benefit from choosing parasi te-free mates if such males provide better parental care, do not trans mit contagious parasites, or provide resistance genes to offspring. Th ere is evidence in support of each of these mechanisms. The immunocomp etence handicap hypothesis posits that secondary sexual characters rel iably reveal the ability of males to resist parasites due to the immun osuppressive effects of testosterone and other biochemicals. Several a spects of these negative feedback mechanisms are supported by laborato ry studies, but evidence from free-living animals is almost completely absent. Corticosterone rather than testosterone may potentially media te the immunocompetence handicap mechanism. A simple version of the im munocompetence handicap is developed suggesting that body condition of male hosts is a sufficient mediator of the handicap mechanism of reli able sexual signaling. Sexual selection appears to be more intense in sexually dichromatic bird species, and comparative studies using pairw ise comparisons of closely related taxa reveal that sexually dichromat ic bird species have larger spleens, larger bursa of Fabricius, and hi gher concentrations of leukocytes than monochromatic species. Parasite -mediated sexual selection is proposed to affect parasite biology by i ncreasing (1) the variance-to-mean ratio in parasite abundance, (2) va riance in the intensity of natural selection affecting hosts, and (3) speciation rates among parasites exploiting hosts subject to intense s exual selection as compared to those subject to less intense selection .