Androgen effects on the development of the zebra finch song system.

In adult zebra finches, males sing and females do not. This behavioral sex difference is associated with sex differences in brain regions that control singing. Treatment of hatchling females with androgen causes permanent masculinization of neuron number in the robust nucleus of the archistriatum (RA), a brain region involved in song. We have re-examined the possible permanent effects of androgen on the zebra finch song system by treating male and female hatchlings with Silastic implants containing 50 or 200 micrograms 5 alpha-dihydrotestosterone (DHT) or 200 micrograms of the antiandrogen flutamide. Birds were sacrificed as adults and their brains prepared for histological analysis of the vocal control nuclei. Some DHT-treated females also received testosterone in adulthood. DHT had small, but significant effects on several attributes of the female song system, including increases of neuron number in RA and neuron size of RA and lateral magnocellular nucleus of the neostriatum (IMAN). DHT had no significant effects in males. Flutamide did not demasculinize any measures of the male song system but hypermasculinized volume and neuron number in RA. These data confirm studies that report few masculinizing effects of androgen on the female zebra finch song system. Nevertheless, the modest effects of DHT leave open the possibility that masculinization of the song system requires an action of androgen, perhaps in synergy with estrogen.