Breeding Males, But Not Females, Have Elevated Androgen Receptor Expression in the Northern House Wren (Troglodytes Aedon), a Temperate Songbird with Female Song
In many male temperate-breeding songbirds, increased plasma testosterone in the early breeding season regenerates song control nuclei that regulate song. Females of some temperate species also sing but have lower circulating testosterone concentrations. We hypothesized that upregulation of steroid receptors in females could compensate for low circulating testosterone, focusing on the northern house wren (Troglodytes aedon), a temperate-breeding songbird in which both sexes sing. We collected brain tissue from both sexes during the early breeding, late breeding, and nonbreeding season. Using quantitative PCR, we quantified mRNA expression of four genes--androgen receptor (AR), estrogen receptors (ER and ER{beta}), and aromatase (AROM)--in three song nuclei--HVC, Area X, and RA--and compared sexes. We found females had lower expression than males of AR, AROM and ER in most song nuclei, especially of AR in HVC in the breeding season. Both sexes, however, had low ER expression in Area X and RA. In males, expression differed seasonally: breeding males had higher expression of AR in RA and AROM in HVC than nonbreeding males. In both sexes, expression differed among song nuclei: in most genes, HVC had the highest expression, followed by RA, then Area X. These findings suggest singing female house wrens do not compensate for low plasma testosterone by upregulating steroid receptors beyond male levels or during early breeding, when they sing most. Conversely, increased AR expression in breeding males indicates differences in the mechanisms regulating male and female song, with testosterone playing a greater role in male birdsong.