Opposing hormonal mechanisms of aggression revealed through short-lived testosterone manipulations and multiple winning experiences.

Territorial aggression is influenced by many social and environmental factors. Since aggression is a costly behavior, individuals should account for multiple factors such as population density or reproductive status before engaging in aggression. Previous work has shown that male California mice (Peromyscus californicus) respond to winning aggressive encounters by initiating aggression more quickly in future encounters, and we investigated the physiological basis for this effect. We found that injections that produced a transient increase in testosterone (T) following an aggressive encounter caused males to behave more aggressively in an encounter the following day. Experience alone was not enough to change aggression, as males treated with saline injections showed no change in aggression. The effect of T injections on aggression was androgen-based, as the inhibition of aromatase did not block the T injections from increasing aggression. Aromatase inhibition did, however, increase aggression in the initial aggression tests (before application of T or saline injections), and aromatase activity in the bed nucleus of the stria terminalis (BNST) was negatively correlated with aggression. A previous study suggested that aromatase activity in the BNST decreases after males become fathers. Thus, distinct neuroendocrine mechanisms allow male California mice to adjust aggressive behavior in response to changes in social and reproductive status.