Steroid hormones and immune function: Experimental studies in wild and captive dark-eyed juncos (Junco hyemalis)

Monogamous and polygynous male songbirds generally differ in their breeding season profiles of circulating testosterone. Testosterone level spikes ear ly in the breeding season of monogamists and then declines, but it remains high in polygynists. Male dark-eyed juncos (Junco hyemalis) are socially mo nogamous and exhibit the usual pattern, but experimental maintenance of hig h testosterone throughout the breeding season alters normal behavior and ph ysiology and affects various components of annual reproductive success but not overall annual success. Because stabilizing selection predicts that alt eration of naturally existing phenotypes should reduce lifetime reproductiv e success, we asked whether prolonged testosterone exposure might impair im mune function and perhaps thereby reduce life span. We assessed immune func tion in captive and wild male juncos that we treated with either testostero ne-filled or empty Silastic implants. Results indicate that prolonged eleva tion of testosterone suppresses antibody production in captive males and ce ll-mediated immunity in wild males. Together these results suggest that tes tosterone-treated males maybe more susceptible to disease or parasitic infe ction. As earlier studies have shown, levels of corticosterone as well as t estosterone are higher in testosterone-treated males, so it is unclear whet her the immune suppression we observed is due to testosterone's direct effe cts on immunity or testosterone's influence on glucocorticoid production. W e discuss results in the context of recent hypotheses regarding life-histor y theory and potential endocrine-immune interactions.