Testosterone in Female Songbirds: Natural, Sexual, and Correlated Responses to Selection

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Males and females are similar in some ways and different in others. Differences may arise because one sex is subject to greater reproductive competition (sexual selection) or because the sexes differ in their ecology (natural selection). Because males and females share a common genome, they are also subject to correlated responses to selection, which occur when genetic variants favored by selection in one sex ('like father, like son') are carried along in the other sex ('like father, like daughter'). Correlated responses may serve as an evolutionary constraint if the attributes that are beneficial to one sex are disadvantageous to the other sex. Steroid hormones are among the proximate mechanisms that lead to sex differences. They may act early in development to organize tissues or later in life to activate trait expression. When hormonal effects are similar in males and females, there is potential for correlated responses to selection. The focus of this proposal is the steroid hormone testosterone (T) and its integrating effect on sex differences in adult males and females of a songbird species, the dark-eyed junco. The research has four objectives. The first is to experimentally elevate testosterone (T) in females and measure its effect on the female's phenotype (e.g. appearance, behavior). The second is to determine whether elevated T is adaptive or maladaptive in females. The third is to assess the potential for correlated responses within and between the sexes by measuring change in T after exposure to a fixed dose of a hormonal stimulant (GnRH). The fourth is to assess sexual independence by comparing the role of T in the activation of aggressive behavior in males and females. Research described in this proposal will add to an already impressive 20-year database, provide opportunities for training future scientists of diverse backgrounds in the conduct of cybernetics, and laboratory and field experiments that address the integration of proximate and ultimate explanations for biological differences. This extensive database will provide fodder for continued synthesis, compilation, and cyber exploitation, and will make a significant contribution to this principle investigator's work, her collaborators, and others who will have access to this gold mine. This cyber research program will also enrich courses taught at Indiana University and elsewhere. Potential societal implications of the findings to be expected include: (1) the impact of hormonally active agents in the environment; and (2) a greater understanding of the relationship between sex and gender.