Lateralization and effects of adult androgen in a sexually dimorphic neuromuscular system controlling song in zebra finches.

Unlike many other sexually dimorphic systems, gonadal secretions do not explain sex differences in the morphology of the telencephalic song control nuclei of zebra finches. It is important to understand whether a novel mechanism for controlling structure is restricted to the forebrain regions specialized for song, and whether other areas respond more typically to gonadal steroids. Therefore, the effects of sex and adult androgen manipulation on the neuromuscular end of the song control system (tracheosyringeal portion of the hypoglossal nucleus, nXIIts, and the syrinx, or vocal organ) were investigated. Because lesion and axotomy experiments indicate a right-side bias in the functional control of song, asymmetry in the motor nucleus and in the ventralis and dorsalis syrinx muscles was also tested. Male-biased dimorphisms existed in the volume of nXIIts, and in syrinx mass and size of muscle fibers, but not in motoneuron number or size. Asymmetries favoring the right side were detected in nXIIts volume and motoneuron number in males, as well as in ventralis and dorsalis fiber size in both sexes. Hormone manipulations had no effect on nXIIts size, neuron size, or number. Testosterone treatment of adult females increased all of the syringeal measures, but the only effect of flutamide in males was to decrease syrinx weight. Thus, male-biased sexual dimorphisms and right side dominance in both nXIIts and the syrinx may facilitate singing behavior. Adult androgen exposure can induce partial masculinization of the syrinx, but other factors must be important in mediating the sex differences in both that structure and the volume of nXIIts.