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The spinal nucleus of the bulbocavernosus (SNB) contains many more motoneurons in adult male rats than in females. Androgens establish this sex difference during a critical perinatal period, which coincides with normally occurring cell death in the SNB region. Sex differences in SNB motoneuron number arise primarily because motoneuron loss is greater in females than in males during the early postnatal period. Perinatal androgen treatment in females attenuates cell death in the SNB region, reducing motoneuron loss to levels typical of males. The results suggest that steroid hormones determine sex differences in neuron number by regulating normally occurring cell death and that the timing of this cell death may therefore define critical periods for steroid effects on neuron number.

Many birds learn song during a restricted 'sensitive' period. Juveniles memorize a song model, and then learn the pattern of muscle contractions necessary to reproduce the song. Of the neural changes accompanying avian song learning, perhaps the most remarkable is the production of new neurons which are inserted into the hyperstriatum ventralis pars caudalis (HVc), a region critical for song production. We report here that in young male zebra finches many of the new neurons incorporated into the HVc innervate the robust nucleus of the archistriatum (RA) which projects to motor neurons controlling the vocal musculature. Furthermore, far fewer of these new neurons are incorporated into the HVc of either adult males that are beyond the sensitive learning period, or young females (who do not develop song). Thus, a major portion of the vocal motor pathway is actually created during song learning. This may enable early sensory experience and vocal practice to not only modify existing neuronal circuits, but also shape the insertion and initial synaptic contacts of neurons controlling adult song.

In zebra finches only males sing, and several nuclei controlling song contain more neurons in adult males than in females. The ontogeny of sex differences in neuron number differs across song regions and overlaps with song learning in males. We examined the development of neuron number in several song regions in both sexes. We then determined whether neurons are born and incorporated into song nuclei as sex differences in neuron number emerge, and whether sex or regional differences in the insertion of such neurons may account for differences in the development of these areas. Males add neurons to hyperstriatum ventralis pars caudalis (HVc) and Area X between 20 and 55 d of age. In females there is no change in HVc neuron number during this time, and Area X never appears as a distinct nucleus. In both sexes, 3H-thymidine administration between 20 and 30 d results in neuronal labeling at 55 d in HVc and the region of Area X. However, in these areas the incidence of labeled neurons is higher in males than in females. In contrast to HVc and Area X, sex differences in neuron number in the robustus nucleus of the archistriatum (RA) and the magnocellular nucleus of the neostriatum (MAN) emerge because males retain neurons that are lost in females between 20 and 55 d of age. Accordingly, RA and MAN neurons are not labeled following 3H-thymidine administration between 20 and 30 d of age. These data indicate that sex and regional differences in the ontogeny of song nuclei are related to differences in the incorporation of neurons born during song learning.