Circulating testosterone is not associated with social challenge or aggression in female northern house wrens (Troglodytes aedon)

Mechanisms that coordinate appropriate behavioral responses to social competition are important for both sexes. Males in many species show a seasonal peak in baseline circulating testosterone (T) concentrations at times when social challenges are more frequent with additional acute elevation in T following competitive interactions. Females in some species show the same patterns, while others do not, making females interesting test cases to evaluate these mechanisms. Here we used three approaches to investigate the interrelationship among circulating T, social challenge, and aggression in female northern house wrens (Troglodytes aedon), a species where female aggression confers a variety of fitness benefits. First, we compared circulating T concentration after a simulated territorial intrusion to T in unchallenged females to determine if T responds to social challenge. Second, we compared T concentrations between females that responded to simulated female intrusions with different levels of aggressive intensity to determine if aggressive behavior relates to T concentrations. Third, we compared natural T concentrations across breeding stages to determine if T is elevated during stages when competition is most common. Overall, female circulating T concentrations were quite low. Neither social challenge nor breeding stage had any impact on T concentrations. Likewise, the intensity of a female's aggressive response to simulated female intruders was unrelated to T. Our results indicate that house wrens may be one of a growing number of species where females mediate competitive behavior through alternative circulating signals or local mechanisms at target tissues.