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birds, there is a sexual dimorphism of both neural and
muscular components related to song production
(Kelley, 1996). However, a female vocal behavior,
termed rapping , is thought to trigger the entire copula-
tory repertoire (Tobias et al., 1998). When the female
frog is unreceptive, it produces a ticking sound, but
when it is ready to lay eggs, it begins to rap. This call
stimulates males to vocalize even more vigorously and
to attempt copulation. It is not yet known what the
neural basis of ticking and rapping is, or whether the
female brain becomes specialized for this behavior
during development.
Female + Testes
Female + Estradiol
The simple hypothesis, then, is that testosterone is
secreted by the testes, and this leads to a masculinized
nervous system in male animals. A number of observa-
tions in birds and frogs suggest that other factors are
involved (Wade and Arnold, 1996; Kelley, 1997). They
raise the possibility that female and male brains differ
from one another even in the absence of gonadal
signals. First, the level of estradiol required to mas-
culinize the nervous system of female birds is quite
high, and even these high levels do not result in a fully
masculinized phenotype. In frogs, the level of circulat-
ing androgen is quite similar in male and female
animals during development. Second, it has not been
possible to block masculine development of the
nervous system in male birds by manipulations
designed to decrease estrogen. Third, when genetic
female zebra finches are pharmacologically engineered
to develop with testes, and with little to no ovarian
tissue, their vocal conrol nuclei continue to have a
female phenotype (Figure 10.20). Finally, female frogs
that receive transplanted testes have a larger larynx
and more laryngeal motor neurons than do females
that are treated with a single androgen (Watson et al.,
These results suggest either that the gonads are a
more complicated endocrine organ than we suspect, or
that the nervous system contains intrinsic signals that
bias its development in the absence of gonadal signals.
For example, some rat diencephalic neurons express a
sex-specific phenotype in vitro (Figure 10.21). When
explanted at E14-17, before the initial surge of testos-
terone, tyrosine hydroxylase-expressing neurons are
30% larger in males, and the number of prolactin-
expressing neurons is two to three times greater in
female tissue, similar to adult animals (Kolbinger et al.,
1991; Beyer et al., 1992). A similar experiment was per-
formed in mice with a Y chromosome that lacks the Sry
FIGURE 10.20 Sexual dimorphism in song production nuclei in
birds. A sagittal section through the brain of song birds shows major
nuclei involved in the learning and production of vocalizations. The
pathway from HVc to RA to nXII is the primary output pathway to
the song production apparatus (bottom). Both HVc (red) and RA
(yellow) are much larger in adult male birds (top left) compared to
adult females (top right). In addition, neurons in the male RA
nucleus have a more elaborate dendritic architecture compared to
those in females. Female zebra finches can be engineered to develop
testes and little ovarian tissue, yet their HVc and RA nuclei do not
become larger (middle left). In contrast, when female birds are
treated with estradiol during development, the HVc and RA nuclei
do become masculinized (middle right). (Adapted from Schlinger,
and these structures are much larger in males (Figure
10.20). Furthermore, when hatchling females are
treated with estradiol, they can grow up to sing almost
as adeptly as male birds (Gurney and Konishi, 1980;
Simpson and Vicario, 1991). In male canaries, the size
of vocal control nuclei changes during the course of a
single breeding season, getting larger as testosterone
levels rise (Nottebohm, 1981). Hormone treatment can
apparently enhance the size of brain nuclei both by
increasing afferent innervation and promoting den-
dritic growth.
It seems odd that females do not vocalize, if only to
facilitate the mating process. In fact, female tropical
wrens do sing a “duet” with the males. Furthermore,
when the song repertoire of a female wren is relatively
large, then the size of its song-control nuclei is similar
to that of males (Brenowitz and Arnold, 1986). The
vocal repertoire of the Xenopus females is also impor-
tant in guaranteeing fertilized eggs. In this species, the
male mating call has been well-characterized, and, like
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