Meeting Abstract
Testosterone (T) is considered a key mediator of the trade-off between mating and parental effort, and as such, decreasing T as the breeding season progresses may be adaptive. Prior work has provided insight into the ‘top-down’ hypothalamic control of this seasonal plasticity; yet, emerging evidence suggests that peripheral mechanisms could also be important. Local regulation of T in peripheral tissues may be especially critical for females, as it would allow them to escape the costs of systemically elevated T while enabling T-mediated traits, like aggression. To begin testing this hypothesis, we measured aggression, plasma T, and peripheral gene expression across the breeding season in female tree swallows, a songbird with intense competition and T-mediated aggression. We focused on the gonad and liver for their role in T production and metabolism, respectively, and the pectoralis muscle for its role in aggression; i.e. because nest defense occurs in flight. We found that females had elevated T during early breeding, but T declined during incubation. Gene expression for ovarian steroidogenic enzymes and hepatic steroid-metabolizing enzymes also significantly declined. In contrast, aggressive response to a simulated intruder remained relatively robust until chick rearing, suggesting that T levels cannot fully explain variation in aggression. However, steroidogenesis and sensitivity to T in the pectoralis did not compensate for declining T, implying that enhanced T processing within this performance-related tissue does not facilitate aggression during incubation. Instead, other mechanisms, like enhanced neurosteroidogenesis or neural T sensitivity, could apply. Collectively, these data highlight important gene regulatory mechanisms that may underlie hormonal and behavioral plasticity in females.
