KETTERSON, E.D.*; NOLAN, Jr., V.; CASTO, J.M.; NEUDORF, D.L.; GRINDSTAFF, J.L.; PARKER-RENGA, I.: Using Hormones to Explore Adaptation and Constraint in a Songbird

We take an experimental approach to the evolution of life histories and the nature of adaptation and constraint. For the past 13 years, we have manipulated plasma levels of testosterone (T) in free-living male dark-eyed juncos (Junco hyemalis), a socially monogamous songbird. Males caught in early spring receive subcutaneous silastic implants that are empty (C-males) or packed with T (T-males). The dose of T simulates the maximum physiological level exhibited by unmanipulated free-living males prior to breeding. The goals have been (1) to document testosterone’s diverse effects on a wide array of phenotypic traits and (2) to relate affected traits to fitness in order to enhance understanding of the evolution of complex adaptations. Behaviorally, T-males sing more, have larger home ranges, and are more attractive to females, but feed their young less often. Physiologically, T-males have higher levels of corticosterone and corticosterone binding globulin, respond more strongly to stressors, and show signs of suppressed immune function. Overall, T-males devote more effort to seeking mates and less effort to parental behavior and self-maintenance. With respect to fitness, T-males sire more young with their social mates and the mates of other males, but are less successful at rearing young at home. Recent work has focused on (1) whether phenotypic effects of T extend to associates of treated males (mates and offspring) as reflected in egg dimensions and contents, offspring sex ratio, or immune status, and (2) potential consequences for females and males of a correlated response in females to selection favoring higher T in males.