Sexual dimorphism in the avian brain is not limited to the song system of songbirds: a morphometric analysis of the brain of the quail (Coturnix japonica).

Nissl-stained sections of adult male and female quail brains were examined for possible sex differences in the sizes of nuclei implicated in reproductive function or known to be dimorphic in songbirds, including nucleus magnocellularis preopticus, two additional preoptic nuclei, n. ventromedialis hypothalami, n. taeniae, n. hypoglossus and n. commissurae pallii. There was a significant sex difference in the volume of one of the preoptic nuclei (which was 38% larger in males), known to be a critical area for the hormonal activation of male copulatory behavior, which cannot be accounted for by overall brain size dimorphism. It is suggested that this nucleus be named nucleus preopticus medianus (POMn). There was no sex difference in the average neuronal density or cross-sectional soma area within POMn; rather, the male POMn consists of a greater total number of neurons than the female POMn. No other nuclei that were measured were dimorphic in volume, including n. hypoglossus. There was small but significant lateral asymmetry in n. commissurae pallii, the left side being about 10% larger in volume.