Meeting Abstract

46.4  Tuesday, Jan. 5  A novel cost of a sexually selected trait in females: more aggressive female tree swallows incubate less ROSVALL, K.A.; Indiana University, Bloomington krosvall@indiana.edu

Among the most studied behavioral trade-offs is that between mating and parental effort, where individuals divide limited resources between attracting mates or repelling rivals on the one hand, and caring for offspring on the other. Historically, the two sexes were placed at opposite ends of this trade-off, with males investing primarily in mating effort and females investing primarily in parental effort. More recently, research shows that females also display exaggerated traits and behaviors that may be important for mating effort. Intrasexual aggression in female tree swallows (Tachycineta bicolor) is one such potentially sexually selected trait: more aggressive females are more likely to obtain a nesting cavity, a limited resource required for reproductive success. High levels of aggression may prove costly, however, if females trade-off aggressive and parental behaviors. Males and females in many species trade-off aggression and provisioning, although much less is known about the potential trade-off between aggression and incubation. Here, I treated female tree swallows with exogenous testosterone (T) to test whether females trade-off intrasexual aggression and incubation behavior. I used a behavioral bioassay to measure female aggressive response to a simulated same-sex intruder. I measured female incubation behavior using iButton data-loggers that record nest temperature from the perspective of an egg. T-implanted females were significantly more aggressive and incubated significantly less than control females. These results experimentally demonstrate that T at least partly mediates aggression in female tree swallows and that females trade-off aggressive and incubation behaviors. Results will be discussed in light of their implications for the evolution of intrasexual aggression in females.