SECONDARY SEXUAL CHARACTERS, PARASITES AND TESTOSTERONE IN THE BARN SWALLOW, HIRUNDO-RUSTICA

The immune-competence hypothesis predicts that the expression of secondary sex traits should be positively related to testosterone levels, but that androgens should simultaneously have negative effects on the immune defence system. Relatively high levels of circulating androgens should therefore result in elevated levels of parasite infections. Parasite load and testosterone levels will be uncorrelated, or even negatively correlated, however, if males with high testosterone levels and large secondary sexual characters are reliable advertisers of genetic resistance to parasites. In male barn swallows, testosterone concentrations peaked during the pre-laying period. Males that had high levels of testosterone (adjusted for the stage of the breeding cycle) at one stage of the breeding cycle also had relatively high levels at other stages. The length of the outermost tail feathers, which are secondary sexual characters currently involved in sexual selection, was positively related to adjusted circulating levels of testosterone, even when potentially confounding variables were controlled statistically. The prevalence (proportion of hosts infected) and intensity (number of parasites per host individual) of ecto-parasites (two species of Mallophaga, one species of Acari) infecting barn swallows were unrelated to adjusted testosterone concentrations. Intensities of ecto-parasite infections were negatively related to tail length of male barn swallows, and unmated males had higher intensities of infections than mated males. These results are consistent with the immune-competence version of the handicap principle.