SEXUAL-DIFFERENTIATION OF BRAIN AND BEHAVIOR - THE ZEBRA FINCH IS NOTJUST A FLYING RAT

In rats and other mammals, sex differences in the brain and behavior r esult from differential secretions of gonadal steroid hormones during early critical periods of neural development. We review the experiment al results that support current ideas about the mechanisms of sexual d ifferentiation in mammals, and then apply the same experimental analys is to the study of sexual differentiation of the neural song circuit i n the zebra finch (Poephila guttata), a passerine song bird. Administr ation of estrogen to young female zebra finches causes the female to d evelop a more masculine song system and. to sing as an adult. This est rogenic masculinization is similar to that found for copulatory behavi or in mammals. However, striking differences emerge in other aspects o f the sexual differentiation process. Experiments that use endocrine a gents to block the masculine development in genetic males have so far failed. Moreover, the brain of zebra finches has an unusually high exp ression of aromatase (estrogen synthetase) in the telencephalon, and e strogen synthesized in. the brain from androgen is released into the g eneral circulation. These results suggest that the brain is the primar y source of estrogen in the body. If so, then a further understanding of sexual differentiation requires more information on the factors tha t regulate the cerebral synthesis of estrogen.