behavioral effects include sex differences in number of laryngeal motor
neurons, differences in number and dendritic extent of N. IX–X interneurons, and differences in synaptic connectivity; for example, the connections
between DTAM and N. IX–X (reviewed in Kelley 1996; Fig. 6.7). In addition, biophysical properties of motor neurons and interneurons may differ
in the sexes in ways that contribute to differences in the generation of vocal
patterns.
4.3. Rodents
Some rodents, such as Mongolian gerbils, produce sexually differentiated
vocalizations (Holman and Hutchison 1982). Steroid-sensitive, vocal
courtship behavior is a function of a specific hypothalamic nucleus, the sexually dimorphic area pars compacta (SDApc) in the male adult gerbil. Sexrelated differences in the number of neurons in this nucleus are apparent
immediately after birth. The differentiation of the SDApc and subsequent
vocalization result from the organizational effect of androgens. Although
female Mongolian gerbils normally do not produce male-typical vocalizations, androgen treatment of a neonatal female leads to male-like vocal
production. The treatment also masculinizes the structure of the brain;
androgen-treated females acquire lateralized SDApc’s that are larger than
those of control females. Cell death and proliferation occur simultaneously
in the neonatal gerbil brain. There is a lower incidence of cell death, occurring earlier in males than in females (Holman et al. 1995). The size and
number of cells in the SDApc can be masculinized by androgen implantation in neonatal females. Androgens also increase laterality in the nucleus;
cell number in the left SDApc is correlated with vocal behaviors (Holman
and Rice 1996). Androgen thus may organize the brain of males by increasing the survival of interneurons in a laterally asymmetric hypothalamic
nucleus related to courtship vocal behaviors.
4.4. Fish
The plainfin midshipman fish produces vocalizations by contracting and
relaxing sonic muscles attached to the lateral walls of the swim bladder
(Cohen and Winn 1967; Bass and Marchaterre 1989). Corresponding to the
vocal differences between type I males and nonhumming type II males and
females, the muscle mass of type I males is six times larger than that of type
II males or of females when scaled to body size, and it contains 3–5 times
more muscle fibers with larger diameters (Bass and Marchaterre 1989).
Moreover, type I myofibrils have enlarged peripheral and central zones of
sarcoplasm that are densely filled with mitochondria, a presumed adaptation for the metabolic requirement for the vocal production (Bass and
Marchaterre 1989). These peripheral differences are likely to be the result
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A. Yamaguchi and D.B. Kelley
neurons, differences in number and dendritic extent of N. IX–X interneurons, and differences in synaptic connectivity; for example, the connections
between DTAM and N. IX–X (reviewed in Kelley 1996; Fig. 6.7). In addition, biophysical properties of motor neurons and interneurons may differ
in the sexes in ways that contribute to differences in the generation of vocal
patterns.
4.3. Rodents
Some rodents, such as Mongolian gerbils, produce sexually differentiated
vocalizations (Holman and Hutchison 1982). Steroid-sensitive, vocal
courtship behavior is a function of a specific hypothalamic nucleus, the sexually dimorphic area pars compacta (SDApc) in the male adult gerbil. Sexrelated differences in the number of neurons in this nucleus are apparent
immediately after birth. The differentiation of the SDApc and subsequent
vocalization result from the organizational effect of androgens. Although
female Mongolian gerbils normally do not produce male-typical vocalizations, androgen treatment of a neonatal female leads to male-like vocal
production. The treatment also masculinizes the structure of the brain;
androgen-treated females acquire lateralized SDApc’s that are larger than
those of control females. Cell death and proliferation occur simultaneously
in the neonatal gerbil brain. There is a lower incidence of cell death, occurring earlier in males than in females (Holman et al. 1995). The size and
number of cells in the SDApc can be masculinized by androgen implantation in neonatal females. Androgens also increase laterality in the nucleus;
cell number in the left SDApc is correlated with vocal behaviors (Holman
and Rice 1996). Androgen thus may organize the brain of males by increasing the survival of interneurons in a laterally asymmetric hypothalamic
nucleus related to courtship vocal behaviors.
4.4. Fish
The plainfin midshipman fish produces vocalizations by contracting and
relaxing sonic muscles attached to the lateral walls of the swim bladder
(Cohen and Winn 1967; Bass and Marchaterre 1989). Corresponding to the
vocal differences between type I males and nonhumming type II males and
females, the muscle mass of type I males is six times larger than that of type
II males or of females when scaled to body size, and it contains 3–5 times
more muscle fibers with larger diameters (Bass and Marchaterre 1989).
Moreover, type I myofibrils have enlarged peripheral and central zones of
sarcoplasm that are densely filled with mitochondria, a presumed adaptation for the metabolic requirement for the vocal production (Bass and
Marchaterre 1989). These peripheral differences are likely to be the result
306
A. Yamaguchi and D.B. Kelley