Post-hatching hormonal modulation of a sexually dimorphic neuromuscular system controlling song in zebra finches.

Sexual dimorphisms are present throughout the zebra finch song system, from forebrain centers to the tracheosyringeal portion of the hypoglossal nucleus (nXIIts) to the muscles of the syrinx (vocal organ). In females, gonadal steroids administered during development can partially masculinize the telencephalic areas, and in adulthood can increase the size of syrinx muscles. In the present study, two experiments were designed to investigate the role of early androgen and estrogen in the development of nXIIts and the ventralis and dorsalis muscles of the syrinx. In experiment one, males and females were treated with testosterone, estradiol, dihydrotestosterone, the anti-androgen flutamide, or a vehicle control for 21 days after hatching. At day 60, nXIIts volume, motoneuron soma size and number were assessed, as well as syrinx weight and the size of ventralis and dorsalis fibers. In experiment two, animals were administered either the estrogen synthesis inhibitor, fadrozole, or vehicle, and the syrinx measurements were taken at day 60. Male-biased sex differences were detected on all measures in both experiments, and several right-side biases were detected. In females, dihydrotestosterone masculinized soma size in nXIIts and testosterone slightly increased syrinx weight. E2 feminized the syrinx of males. However, flutamide did not prevent masculine development of either structure, and fadrozole did not inhibit feminine syrinx development. These results are consistent with the idea that, as in the forebrain, steroid hormones can stimulate aspects of sexual differentiation, but they may not be direct triggers for the process.