Steroid accumulation in song nuclei of a sexually dimorphic duetting bird, the rufous and white wren.

This study tested the hypothesis that the relative proportion of neurons that are hormone sensitive in avian song control nuclei is related to the basic motor ability to sing, whereas the absolute number of such neurons is related to the complexity of song behavior. Either [3H]testosterone (T) or estradiol (E2) was injected into male and female rufous and white wrens (Thryothorus rufalbus), a tropical species in which females sing duets with males but have smaller song repertoires than males. Autoradiographic analysis indicated that there were no sex differences in the proportions of T or E2 target cells in two song nuclei: the high vocal center (HVC) and the lateral portion of the magnocellular nucleus of the anterior neostriatum (IMAN). The density of labeled cells per unit volume of tissue did not differ between the sexes in either song nucleus. Males have larger song nuclei, however, which is consistent with their more complex song behavior, and therefore have a greater total number of hormone-sensitive neurons in these regions than do females. Comparison of these results with measures of hormone accumulation in zebra finches, canaries, and bay wrens supports the hypothesis presented.