The bi-directional link between hormones and behavior has been a rich area of research for decades, perhaps especially so for the vertebrate sex steroid hormone testosterone (T). Theory on the evolution of T was advanced by the “challenge hypothesis,” which presented a framework for understanding interspecific, seasonal, and social variation in T levels in males, and how they are shaped by the competing demands of parental care and male-male competition (Wingfield et al. 1990). My lab has been working to understand how this works in females, using the tree swallow as a model. In this system, female aggression is adaptive during territorial establishment (Rosvall 2008) and mediated by T (Rosvall 2013). However, seasonal changes in aggression do not mirror changes in ovarian T production (George & Rosvall 2018; Bentz et al. in review), suggesting that other mechanisms must promote aggression when T is low. Our findings thus far suggest that androgens may promote aggression in females, but that selection may favor more finely-tuned processing of T at the tissue level, perhaps as an adaptive mechanism to minimize the costs of T.