Gonadal hormones induce dendritic growth in the adult avian brain.

Ovariectomized adult female canaries were treated with physiological doses of testosterone, dihydrotestosterone, or estradiol. Singing, which is typical of males, occurred in the testosterone-treated birds but not in any of the other birds. The effect of these hormones was assessed on dendrites from a class of neurons in the nucleus robustus archistriatalis (RA), a forebrain nucleus for song control. The RA neurons of the testosterone-treated birds had dendritic trees resembling those of intact males. The RA neurons of the estradiol- and dihydrotesterone-treated birds resembled those of intact females. All hormone-treated groups had dendrites that were significantly longer than those of untreated ovariectomized females. Thus gonadal hormones induce dendritic growth in the adult avian brain.