Plasma sex steroids and tissue aromatization in hatchling zebra finches: implications for the sexual differentiation of singing behavior.

TLDR: The puzzle remains why the song system is not masculinized in females, who possess high levels of aromatizable androgens and telencephalic aromatase, as in nonpasserine birds.

Abstract: One of the best examples for sex hormone regulation of brain development is found in songbirds. In zebra finches, only males sing because of striking sex differences in the neural circuitry that controls songs. Because developing females treated with estradiol (E2) develop a masculine song system, E2 is considered the normal masculinizing hormone. However, questions about the role of E2 in male development persist, because E2 treatments that masculinize song can demasculinize other sexual behaviors, and there exists contradictory evidence for high levels of circulating E2 in developing males. We remeasured plasma steriods in zebra finches during the first 13 days after hatching. E2 circulated at low levels, and there were no sex differences in circulating E2, estrone, testosterone, androstenedione, or dihydrotestosterone. We also measured aromatase activity [( 3H]androstenedione conversion to [3H]estrone and [3H]E2) in gonad, adrenal, brain, and other tissues of hatchlings. Aromatase was abundant in ovary...