Abstract
Abstract 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. Testoste...
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