SEXUAL DIMORPHISM IN THE VOLUME OF SONG CONTROL NUCLEI IN EUROPEAN STARLINGS - ASSESSMENT BY A NISSL STAIN AND AUTORADIOGRAPHY FOR MUSCARINIC CHOLINERGIC RECEPTORS

Previous studies have found that the volume of several song control nu clei is larger in male songbirds than in female songbirds. The degree of this volumetric sex difference within a given species appears to be systematically related to the degree of the behavioral sex difference . The largest volumetric differences have been reported in species in which the male sings and the female sings little, if at all, and the s mallest sex differences in volume have been reported in species in whi ch males and females both sing in nearly equal amounts. We compared th e volume of three song control nuclei in male and female European star lings (Sturnus vulgaris), a species in which females are known to sing , though at a much lower rate than males. We investigated the volume o f hyperstriatum ventrale, pars caudale, nucleus robustus archistriatal is, and area X of the lobus parolfactorius as defined with the use of a Nissl stain. In addition, we measured the volume of area X as define d by the density of muscarinic cholinergic receptors visualized by in vitro receptor autoradiographic methods. The volumes of all three of t he song nuclei, as defined by Nissl staining, are significantly larger in males than in females. For area X, Nissl staining and receptor aut oradiography indicate the same significant volumetric sex difference. The three nuclei are approximately one and one half to two times large r in males than in females, a degree of dimorphism that is intermediat e to those reported for other species. Previous investigations of sex differences in the avian vocal control system have used only Nissl sta ins to define nuclear volumes. We demonstrate in this paper that recep tor autoradiography can be used to assess dimorphisms in nuclear volum e. Broad application of this approach to a number of neurotransmitter receptor systems will better characterize the dimorphisms in the song system, and therefore will provide greater insight into the neuroanato mical and neurochemical control of birdsong. (C) 1993 Wiley-Liss, Inc.