Hormonal influence on song structure and organization: the role of estrogen.

The development of song in songbirds is a complex phenomenon that involves memory and learning, sensorimotor integration, and neural and muscular maturation. Gonadal hormones are involved in each of these steps, as they influence the differentiation of the neural song system, the incorporation and survival of neurones, and the development of muscles used for song production. In young males the development of song, therefore, is closely linked to the secretion of testosterone by the testicles. Castration results in the development of incomplete or unstable songs, and hormone replacement leads to the development of crystallized or stable song. However, testosterone does not act solely as an androgen. The brain of songbirds contains high concentrations of the enzyme aromatase, which converts testosterone into estradiol. Estradiol then binds to estrogen receptors, which in the song system are found only in the nucleus HVC. This forebrain nucleus, also called the "master nucleus," codes for the syntactic structure of song, i.e. for the particular combination of simple elements-syllables-that characterize the song of an individual. In this paper, we will review our studies on the role of estrogen in guiding the organization of song in canaries.