Date of Award


Document Type

Thesis open access



First Advisor

Troy G. Murphy


Studies on aggression and status signaling have traditionally focused on the male sex. As a result, the function of female aggression and status signaling is not nearly as thoroughly understood as it is in males. Although testosterone is characteristically known as a “male hormone,” recent evidence has suggested that in many species females develop testosterone linked fighting potential and ornamentation. In this thesis, I report the results from an investigation on the influence of testosterone on female dominance and status signaling. The female American goldfinch is aggressive year-round over limited resources such as food, and at times females are observed to be more aggressive than males. Additionally, American goldfinches have a dynamic bill color that has been correlated to testosterone and has been shown to serve as a status signal in females. Females were placed into dyads consisting of a testosterone treated individual and a control treated individual of similar dominance. Behavioral observations were recorded over a monopolizable food source, allowing us to determine which female was dominant within each dyad. Additionally, bill color was measured using a spectrophotometer before and after treatment. Although testosterone treated females won a majority of the trials that took place, testosterone did not significantly predict the outcome of the dyad trials. Interestingly, wing size was an accurate predictor of winning, with over 80% of the winners having larger wings. Therefore, it may be that wing was so tightly correlated with winning that it overshadowed our ability to detect any influence of testosterone on competitive potential. Testosterone did not significantly influence bill color in this study. Taken together, these findings indicate that wing size may be a more important mediator of aggression than testosterone within this species.