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The “immunocompetence handicap” hypothesis predicts that reproductive hormones, in particular testosterone (T), are immunosuppressive and consequently increase susceptibility to diseases and parasite infections, but this prediction has not been evaluated in free-living birds and the factors mediating the immunosuppressive influence of T remain poorly known. To address these issues, we administered supplemental T via implants to free-ranging adult male Dark-eyed Juncos (Junco hyemalis) and characterized the effects of this treatment on infection with the two most prevalent hematozoan parasites in this species, Leucocytozoon fringillinarum and Trypanosoma avium. Males caught at the beginning of their breeding season (May) received T-filled, empty (control), or no Silastic capsules, and were recaptured five weeks later. Capsule implantation had, by itself, no effect on parasite infections, body mass, or size of an androgen-dependent secondary sexual characteristic, the cloacal protuberance. Testosterone treatment maintained physiologically high plasma levels of the steroid for the duration of the study, thus preventing the seasonal decline in these levels that occurred in control males. As predicted by the immunocompetence handicap hypothesis, the hormone treatment increased blood L. fringillinarum abundance. This increase was specific, in that implanted T did not affect (1) either the prevalence or the incidence of L. fringillinarum, (2) body mass, or (3) size of the cloacal protuberance. Trypanosoma avium prevalence was not influenced by T treatment either, but it increased between May and June, which suggests that it is regulated by factors other than the activity of the reproductive system. These findings provide the first demonstration in free-ranging birds that experimentally elevated physiological T levels increase hematozoan infection. El Tratamiento de Testosterona en Machos Silvestres de Junco hyemalis Exacerba la Infección de Hemoparásitos

Testosterone Treatment to Free-Ranging Male Dark-Eyed Juncos (Junco Hyemalis) Exacerbates Hemoparasitic Infection