When the optimal phenotype differs by sex, the evolution of sex differences, or sexual dimorphism, is expected. In many species, however, the degree of dimorphism is mild, and numerous individuals express traits, e.g. body size or behavior, that fall between male and female norms. Females in this group, known as androgynous females, are of interest because they offer a natural experiment for understanding the sources and consequences of sexual dimorphism. Variation in hormone exposure, especially androgens like testosterone, is a primary mechanism for the development of sex differences in vertebrates. This variation may also mediate the expression of androgynous traits within a sex, leading to covariation among male-like traits, e.g. big females may also be aggressive females. If male-like females differ from typical females in survival or reproduction, the degree of sexual dimorphism may reduced or enhanced. This project seeks to determine whether the degree androgyny expressed in one trait (morphology or behavior) is related to the level of androgyny in other traits, which would indicate a common mechanism controlling trait expression. Additionally this project will strive to determine whether androgen exposure during development or in adulthood is driving this covariation, and also whether androgynous females vary from typical females in survival or reproductive success. Answers to these questions will inform our understanding of the potential links between social behavior, morphology and hormone exposure levels in females as well as the evolution of complex traits that are mediated by hormones. This work will be done with free-living songbirds and serve to train undergraduates to conduct research as part of training programs such as NSF's Research Experience for Undergraduates (REU) and Indiana University's Student Achievement in Research and Scholarship (STARS) programs.
