Host immune function and sexual selection in birds

Parasites have been hypothesized to affect sexual selection of their hosts, if secondary sexual characters reliably signal absence of infectious paras ites, superior parenting ability caused by the absence of parasites, or her itable resistance to parasites, for which there is some intraspecific and i nterspecific evidence. Measures of immune defence of hosts provide reliable information on the current infection status of individuals of the chosen s ex, usually males, and correlations between immune defence and development of secondary sexual characters thus provide a novel critical test of parasi te-mediated sexual selection. In a comparative study of birds, sexually dic hromatic species had higher immune defences, measured in terms of leukocyte concentration and the size of spleen and bursa of Fabricius, respectively, than closely related, monochromatic species. Male plumage brightness was c onsistently negatively related to the size of the spleen in males of sexual ly dichromatic species, but not in males of monochromatic species. Hence, t he brightest males, which frequently are preferred as mates by choosy femal es, had low levels of immune defence, suggesting that such males were healt hy. This provides evidence for a general role of parasites in sexual select ion among their bird hosts.