Sexual Differentiation of Brain and Behavior in the Zebra Finch (Poephila guttata): a Cellular Analysis

Male zebra finches sing a brief song phrase to the female during courtship. castration of an adult male reduces the bird's frequency of singing; testosterone replacement reinstates the behavior. Testosterone treatment of female zebra finches does not activate song nor induce other elements of courtship behavior. Correlative changes of brain and behavior in zebra finches are organized by sex hormones during development. Newly hatched zebra finch chicks were subcutaneously implanted with Silastic pellets containing either 50 μg of dihydrotestosterone or 50 μg of 17 β-estradiol. Testosterone treatment activates song in adult females which were implanted with estradiol when chicks, but fails to activate song in those females which had received dihydrotestosterone. The singing females approach a sexual partner with pivoting movements, straighten to an erect posture, fluff their throat feathers, and rapidly repeat their song phrase in a behavioral sequence which closely resembles that of the male. In zebra finches, brain nuclei of the efferent pathway for control of song show dramatic sex differences in their volume (Nottebohm and Arnold; Science 194 1976 211-213). 17 β-estradiol treatment of genetically female chicks organizes male-like cytoarchitectonic differentiation of the telencephalic song nuclei RA, HVc, MAN, and X. Dihydrotestosterone induces masculinization of the brain stem song nuclei nXII and DM. Dendritic field spread, soma size, and the consequent volume of the brain nucleus is larger in males than females at all levels of the song system. The exposure of female chicks to either androgen or estrogen supports growth of the hormone's respective target neurons in either the brain stem or telencephalic song nuclei. These neurons reach a size identical with that of the equivalent cell type in a normal male. Anatomical comparison of normal adult male and female song systems reveals that all cell types and all identified connections are present in both sexes. Thus, the specificiation of cellular identity-i.e., position, dendritic morphology and efferent synaptic projection-is expressed independently of the hormonal environment. Rather than selecting pathways of anatomical differentiation, androgens and estrogens exert a similar pleiotrophic effect on their respective target neurons. Although in the female song system we identify all the cell types and connections of the male, testosterone does not activate song. Thus, 17 β-estradiol may also exert a specific inductive effect on the telencephalic song neurons which renders them physiologically competent to respond to testosterone in the adult.