Castro, J.M.*; Parker-Renga, I.M.; Ketterson, E.D.; Val Nolan Jr.: Experimentally elevated testosterone in male dark-eyed juncos suppresses cell-mediated immune function of social mates and offspring.

The immunocompetence handicap hypothesis (ICHH) predicts that testosterone-dependent secondary sexual signals that attract females also serve as honest indicators of male quality owing to testosterone’s suppressive effect on immune function. Only males of high quality can express attractive traits and resist disease despite suppressed immunity. We have previously shown, as predicted by the ICHH, that experimental elevation of testosterone (T) in male dark-eyed juncos suppresses immunity and enhances male attractiveness to females. We have also shown that T suppresses male parental behavior and that females attempt to compensate for reduced male care by increasing their own parental effort. Here we test whether the effects of treating males with T might extend beyond individual males and indirectly influence immune function of their social mates and offspring. We used wing-web swelling in response to an antigenic challenge as a test of cell-mediated immunity. We tested free-living females juncos that were mated to T-treated males (T-males) or controls (C-males), and tested offspring of those matings. We found that immune responses were significantly suppressed in mates of T-males (41% lower) and their offspring (19% lower) as compared to those of C-males. Combining these results with earlier findings, we suggest that the costs to males of T-induced attractiveness may go beyond immunosuppression. Reductions in T-nestling immunity may explain a previous finding that in juncos T-males fledge significantly fewer young from their nests than C-males. Additionally, the consequences for females of choosing males with traits enhanced by T appear to be more complicated than previously thought.