Devin Zysling

Intersexual conflict occurs when a trait that is beneficial to one sex is costly to the other. Testosterone often mediates traits in both males and females to varying degrees. Surprisingly little is known, however, about the role of testosterone in female vertebrates, and whether effects of testosterone in females might constrain the evolution of traits in males. Our goal is to evaluate the potential for intersexual conflict by examining the impact of experimental elevation of testosterone on the behavior and physiology of female dark-eyed juncos.

Past work from my lab and others has shown that experimental elevation of testosterone in male birds can increase male attractiveness to females and male success at extra pair fertilizations. These findings create a paradox. If males could achieve higher fitness through increasing testosterone, what prevents males from having higher levels of testosterone than they already do? One possible explanation may lie in correlated responses in females to selection on males. That is, if selection for higher testosterone in males had harmful consequences for females, male evolution might be constrained.

Natural and sexual selection can act independently on males and females. Importantly, however, selection on one sex can lead to a correlated response in the other and vice versa. If T-mediated traits are regulated through similar mechanisms in males and females, then selection on one sex for higher testosterone could have an impact on the other and thus both sexes need to be taken into account in order to predict evolutionary dynamics. One way to determine whether selection might act in this way would be to elevate testosterone in females, document the consequences, and ask whether these effects are likely to be detrimental or beneficial.

My research has focused mainly on the dark-eyed junco. I elevated testosterone exogenously in order to address two broad questions. First, what are the effects of experimentally elevated testosterone on the phenotype of female juncos and how might these effects relate to fitness? Second, could the evolution of the male phenotype be constrained by the effects testosterone has on females

To summarize, we found that testosterone treated females displayed increased aggression, higher corticosterone response to stressors, and lowered immune response. This is similar to what has been found in previous studies of males of this species and may present costs for females. The impact of testosterone on corticosterone, immune function and aggression is not sex-limited, which demonstrates the potential for correlated responses to selection on males and females. Future studies will address whether the impact on females relates to fitness, but it appears that both sexes are subject to the costs of testosterone, while only males may experience the benefits.

Although more studies must be done addressing direct fitness consequences, the evolution of the male phenotype may be constrained by the effects testosterone has on females.

Zysling, D.A., Greives, T.J., Demas, G.E., Casto, J. and Ketterson, E.D. (2004). The effects of steroid hormones on female behavior and physiology: Evidence for intersexual conflict in the dark-eye junco (Junco hyemalis). Presented at the annual meeting of the Society for Comparative and Integrative Biology, New Orleans, LA.

Greives, T.J., Zysling, D.A., Demas, G.E. and Ketterson, E.D. (2004) Immunological and behavioral responses to experimentally elevated testosterone in female dark-eyed juncos (Junco hyemalis): some evidence for possible constraints on evolution? Presented at the annual meeting of the Society for Comparative and Integrative Biology, New Orleans, LA.