SEX-DIFFERENCES IN THE VOLUME OF AVIAN SONG CONTROL NUCLEI - COMPARATIVE-STUDIES AND THE ISSUE OF BRAIN NUCLEUS DELINEATION

Two goals of research on neural sex differences are to establish the b ehavioral function of such sex differences and to identify precisely w hat features differ between males and females. Comparative studies of sex differences in the volume of brain nuclei within the songbird voca l control circuit provide one way to address these goals. Informative comparisons can be either inter-specific or intra-specific. Inter-spec ific comparisons of species within the songbird suborder allow one to establish how species variation in the degree to which there is a sex difference in nuclear volume relates to species variation in the degre e to which there is a sex difference in vocal behavior. Intraspecific comparisons of sex differences in nuclear volume involve the compariso n of a variety of histochemical methods to define nuclei and describe a nucleus within a species. Sex differences in nuclear volume have now been measured for at least some song control nuclei in 10 different p asserine species. In species with more complex male than female song, the volume of key song control nuclei is on average larger in males th an in females. However, future studies will require more refined measu res of vocal behavior and perceptual abilities to make more precise co rrelations between brain and behavior. In European starlings (Sturnus vulgaris), the volume of the vocal control nucleus, area X was found t o be on average 1.95 times bigger in males than in females based on Ni ssl stained sections. Variation in neurotransmitter receptor density a s determined by quantitative receptor autoradiography can also be used to define clearly the boundaries of a nucleus. When the boundaries of area X in male and female starlings were defined based on variation i n muscarinic cholinergic and a2-adrenergic receptor densities, volumet ric estimates were obtained that are nearly identical to those obtaine d with the use of Nissl stains. Intra-specific comparisons of this sor t extend our knowledge concerning the neurochemical basis of sex diffe rences in nuclear volume. The wide application of this method would gr eatly increase our understanding of neural sex differences.