GREIVES, Timothy J.; ZYSLING, Devin A.; DEMAS, Gregory E. ; KETTERSON, Ellen D.; Indiana University, Bloomington: Immunological and Behavioral Responses to Experimentally Elevated Testosterone in Female Dark-eyed Juncos (Junco hyemalis): some evidence for possible constraints on evolution?

Monogamous songbirds display elevated levels of testosterone (T) prior to nest building, with decreasing levels as reproduction progresses, and a marked difference in concentrations between the sexes. Elevated T in male dark-eyed juncos is an attractive trait to female conspecifics; however evolution of elevated T may be constrained if T imposes fitness costs to females and is genetically correlated between the sexes. We artificially elevated plasma T in females to spring-peak levels, to address the question of fitness costs of T to females by examining both behavior and physiology in a captive population of dark-eyed juncos (Junco hyemalis) during the breeding season. Testosterone-implanted (T-) females displayed a less robust cell-mediated immune response to a challenge of an innocuous protein (PHA) than did controls (C-females), but T- and C-females did not differ in season-long change in mass or condition. In addition, T-females vocalized chacks, a distress signal, more often than C-females, when placed in a novel environment. Treatment had no effect on locomotor activity. When males were added to the novel environment, C-females tended to be more responsive, but not significantly so, and males did not differ in courtship intensity toward T- or C-females. Our findings indicate that elevated T might impose a physiological fitness cost in females by dampening immune responses, without any accompanying fitness gains (or costs) associated with altered activity levels or courtship behavior.