steroids and singing is poor in both sexes (Schwabl and Sonnenschein 1992).
In this species, behavioral interactions between mates, rather than the
endocrine state of the individual, may play a larger role in activating singing
behavior (Schwabl and Sonnenschein 1992). In extreme cases, courtship
singing may be completely independent of T. In male European robins,
blocking the surge of T during the spring by implanting flutamide, an androgen antagonist, did not influence the song production (Schwabl and Kriner
1991). The results suggest that T and its androgenic metabolites do not activate courtship vocalizations in this species, although we cannot rule out the
possibility that T may exert its effect via aromatization into E 2 . In conclusion, the gonadal steroids of males and females activate courtship vocalizations in both sexes although the exact steroid metabolites involved, and
their potency with respect to vocal activation, differ across species.
3.1.1.2. Hormonal Modification of Vocal Organs
Circulating gonadal steroids modify the peripheral organ involved in vocal
production. In gray partridges, Perdix perdix, administering T to both sexes
results in thickening of the external membrane of their vocal organ, the
syrinx, which modifies the acoustic properties of the call (Beani et al. 1995).
The syrinx of adult songbirds is sexually distinct in weight and in the activity of two cholinergic enzymes required for synaptic transmission, choline
acetyltransferase (CAT) and acetylcholinesterase (AChE). These anatomical and molecular differences are regulated by circulating T. Castrating
adult male zebra finches and canaries decreases syringeal weights and
activity of CAT and AChE, whereas administering T or other androgenic
metabolites for 1–4 weeks restores weight and activity of AChE but not of
CAT (Luine et al. 1980, 1983). Ovariectomizing adult female zebra finches
has no effect on the syrinx, but administering T to ovariectomized females
for a month masculinizes the syrinx by increasing syringeal weight and
activity of AChE (Luine et al. 1980, 1983). In addition, the number of acetylcholine receptors in syringeal muscle positively correlates with circulating
levels of T, indicating that neuromuscular transmission at syringeal synapses
is enhanced by T (Bleisch et al. 1984). Thus, the anatomical structure and
physiological function of the vocal organ can be readily modified by
changes in the circulating levels of androgen in adulthood. Whether these
changes in the periphery are necessary for song production in songbirds is
not known.
3.1.1.3. Hormonal Modification of the CNS Associated with Activation of
Courtship Vocalizations in Birds
Hormones that activate courtship vocalizations are shown to bind to the
neurons in the vocal pathway in the CNS and modify their function. The
specific brain regions that mediate vocal production have been identified
particularly well in songbirds (Fig. 6.5). The song-control system consists of
286
A. Yamaguchi and D.B. Kelley
In this species, behavioral interactions between mates, rather than the
endocrine state of the individual, may play a larger role in activating singing
behavior (Schwabl and Sonnenschein 1992). In extreme cases, courtship
singing may be completely independent of T. In male European robins,
blocking the surge of T during the spring by implanting flutamide, an androgen antagonist, did not influence the song production (Schwabl and Kriner
1991). The results suggest that T and its androgenic metabolites do not activate courtship vocalizations in this species, although we cannot rule out the
possibility that T may exert its effect via aromatization into E 2 . In conclusion, the gonadal steroids of males and females activate courtship vocalizations in both sexes although the exact steroid metabolites involved, and
their potency with respect to vocal activation, differ across species.
3.1.1.2. Hormonal Modification of Vocal Organs
Circulating gonadal steroids modify the peripheral organ involved in vocal
production. In gray partridges, Perdix perdix, administering T to both sexes
results in thickening of the external membrane of their vocal organ, the
syrinx, which modifies the acoustic properties of the call (Beani et al. 1995).
The syrinx of adult songbirds is sexually distinct in weight and in the activity of two cholinergic enzymes required for synaptic transmission, choline
acetyltransferase (CAT) and acetylcholinesterase (AChE). These anatomical and molecular differences are regulated by circulating T. Castrating
adult male zebra finches and canaries decreases syringeal weights and
activity of CAT and AChE, whereas administering T or other androgenic
metabolites for 1–4 weeks restores weight and activity of AChE but not of
CAT (Luine et al. 1980, 1983). Ovariectomizing adult female zebra finches
has no effect on the syrinx, but administering T to ovariectomized females
for a month masculinizes the syrinx by increasing syringeal weight and
activity of AChE (Luine et al. 1980, 1983). In addition, the number of acetylcholine receptors in syringeal muscle positively correlates with circulating
levels of T, indicating that neuromuscular transmission at syringeal synapses
is enhanced by T (Bleisch et al. 1984). Thus, the anatomical structure and
physiological function of the vocal organ can be readily modified by
changes in the circulating levels of androgen in adulthood. Whether these
changes in the periphery are necessary for song production in songbirds is
not known.
3.1.1.3. Hormonal Modification of the CNS Associated with Activation of
Courtship Vocalizations in Birds
Hormones that activate courtship vocalizations are shown to bind to the
neurons in the vocal pathway in the CNS and modify their function. The
specific brain regions that mediate vocal production have been identified
particularly well in songbirds (Fig. 6.5). The song-control system consists of
286
A. Yamaguchi and D.B. Kelley