Sex differences in songbirds 25 years later: What have we learned and where do we go?

About 25 years ago, Nottebohm and Arnold reported that there are profound m ale-biased sex differences in volume in selected nuclei in telencephalic po rtions of the song control system. This review focuses on issues related to the cellular bases of these sex differences in volume and comparative stud ies that might elucidate the function of this variation between the sexes. Studies utilizing a variety of neurohistological methods in several differe nt species to define the boundaries of two key telencephalic song nuclei HV c and the robust nucleus of the archistriatum (RA) all tend to find a sex d ifference in volume in agreement with Nissl-defined boundaries. Sex differe nces in volume in nuclei such as HVc and RA are associated with differences in cell size and cell number. Other attributes of the phenotype of cells i n these nuclei are also different in males and females such as the number o f cells expressing androgen receptors. Comparative studies have been employ ed to understand the function of these sex differences in the brain. In som e songbird species, females sing rarely or not at all, and the brain nuclei that control song are many times larger volume in males than females. In o ther species, males and females sing approximately equally, and the brain n uclei that control song are approximately equal between the sexes. Recently , statistical methods have been employed to control for phylogenetic effect s while comparing the co-evolution of traits. This analysis indicates that the evolution of sex differences in song has co-evolved with the evolution of sex differences in singing behavior in songbird species. Future studies should focus on the function of the smaller song control nuclei of females and investigate the role these nuclei might play in perception as well as i n production. Microsc. Res. Tech. 54:327-334, 2001. (C) 2001 Wiley-Liss, In c.