Meeting Abstract
Prior social competition is thought to physiologically prime animals for future confrontations. Research on this phenomenon in vertebrates has largely focused on males and circulating levels of testosterone (T). In recent years, however, it has become increasingly clear that that female-female competition is also widespread and adaptive in many species, but social modulation of T in females may not always be adaptive. To understand female responses to aggressive encounters in their social environment, it is critical to determine whether females are physiologically capable of elevating T. We addressed this question in female tree swallows (Tachycineta bicolor), a songbird with intense female competition for nesting sites. We injected females with gonadotropin-releasing hormone (GnRH) at multiple stages of reproduction, measured T levels in blood collected before and 30 min after injection, and compared these females to saline-treated controls. We found that females were able to elevate T in response to exogenous GnRH early in the breeding season (territory establishment through egg deposition), but not late (incubation and chick-rearing). Baseline T levels also declined over the season, corresponding to a seasonal decline in naturally occurring rates of aggression. These results suggest that social modulation of T may be possible early in the season when aggression is most prevalent, but females cannot elevate T during later stages when elevated T or aggression may interfere with maternal care. As a consequence, these patterns have key implications for understanding how selection shapes mechanisms of behavior in females.
