Meeting Abstract
The challenge hypothesis suggests that variation in testosterone (T) secretion is shaped by trade-offs between competition and parental care in male vertebrates. Female competition is also widespread; however, it is not clear whether or how the challenge hypothesis applies to females. Here, we explore this issue in a series of studies using a seasonal comparative approach with the tree swallow (Tachycineta bicolor), a cavity-nesting bird for which challenges from prospecting rivals pose a threat to territorial females. In this system, more aggressive females are more likely to obtain a nesting cavity, and trade-offs between aggression and maternal care are influenced by T. We show that, during territorial establishment, females are capable of elevating T, and gene expression analyses suggest that they actively produce the potent androgen dihydrotestosterone as well. During incubation, T levels rapidly decline, but seasonal changes in aggression do not mirror these hormonal shifts. Gene expression analyses of muscle and brain suggest that androgen processing may be sustained from territory establishment through incubation, perhaps providing local, tissue-specific mechanisms that may support aggression when systemic T levels are low. These seasonally variable patterns of behavior, hormones, and gene expression hint at potentially adaptive tissue-level mechanisms that may allow females to respond to social competition while also mitigating trade-offs with parental care.
