Androgens and the immunocompetence handicap hypothesis: unraveling direct and indirect pathways of immunosuppression in song sparrows.
The immunocompetence handicap hypothesis proposes that testosterone (T)-dependent sexual signals are honest indicators of male health or genetic quality because only high-quality males are able to withstand the obligate effects of T-induced immunosuppression. In birds, the basic assumption that T suppresses immune function is equivocal, and the physiological mechanisms underlying T-induced immunosuppression remain to be investigated. We explored the proximate pathways of T-induced immunosuppression in song sparrows (Melospiza melodia) by treating captive nonbreeding males with different androgens and measuring several components of acquired immune function. Males implanted with T suppressed cell-mediated and humoral immune responses compared to males implanted with 5alpha-dihydrotestosterone (DHT), dehydroepiandrosterone, or control (empty) implants. Furthermore, T treatment increased plasma levels of corticosterone and decreased body mass and fat stores in relation to other treatments. The failure of DHT to depress immune function suggests that T-induced immunosuppression does not occur through a direct pathway because both T and DHT bind to androgen receptors on target cells. Instead, we outline indirect pathways that are likely responsible for suppression of the avian immune system that include stress-induced immunosuppression, aromatization to estrogen, and alterations in energy allocation that constrain expenditures toward immune system activation.
Department of Biology, University of Washington, Seattle, Washington 98195, USA. nowenash@u.washington.edu
Noah T
NT
Owen-Ashley
Dennis
D
Hasselquist
John C
JC
Wingfield
D000728:Androgens / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D003345:Corticosterone / Q000097:blood; D003687:Dehydroepiandrosterone / Q000494:pharmacology; D013196:Dihydrotestosterone / Q000494:pharmacology; D007121:Immunocompetence; D008297:Male; D046370:Sparrows / Q000276:immunology* / Q000378:metabolism; D013256:Steroids / Q000097:blood; D013739:Testosterone / Q000378:metabolism
D000728:Androgens; D013256:Steroids; D013196:Dihydrotestosterone; D013739:Testosterone; D003687:Dehydroepiandrosterone; D003345:Corticosterone