rubecula), and Gambel’s white-crowned sparrow (Zonotrichia leucophrys
gambelii), males sing in the winter when circulating levels of T are low
(Logan and Wingfield 1990; Schwabl and Kriner 1991; Wingfield 1994a,
1994b). Castrating wild males does not suppress winter aggression in song
sparrows (Wingfield 1994b). Songs of female cardinals are produced while
plasma levels of testosterone are at undetectable levels (Yamaguchi 1996).
These results indicate that aggressive songs can be independent of androgen. However, a recent study suggests that very low levels of endogenous
androgen and estrogen may in fact be capable of activating aggressive song
production. Inhibition of androgen and estrogen using an aromatase
inhibitor and an antiandrogen during the nonbreeding season, when the T
level is very low, decreased the amount of singing and overall aggression in
male song sparrows (Soma et al. 1999b). Because the gonads do not synthesize steroids during nonbreeding season, nongonadal steroidogenesis
and up-regulation of androgen receptors may account for activation of
aggressive song production during this time (Soma et al. 1999b). In
summary, despite its low circulating levels, androgen may still play a role in
activating aggressive vocalizations in songbirds.
In addition to steroid hormones, AVT may also play a role in activating
aggressive vocalizations in some species of birds. In the field sparrow
Spizellas pusilla, males have two acoustically distinct song types during the
breeding season: a simple, multipurpose song that is used both for courtship
and territorial aggression and a complex song that is used predominantly
in male–male aggression. Goodson (1998) showed that the aggressive songs,
but not the multipurpose songs, of males can be induced by intraventricular infusion of AVT. This finding differs from the role of AVT in frogs, where
AVT selectively activates courtship vocalizations without activating aggressive vocalizations (Semsar et al. 1998).
In previous examples reviewed in this chapter, the effects of AVT are
partly dependent on gonadal steroids. However, in the case of aggressive
vocalizations in songbirds, AVT might activate vocalizations independent
of steroids. Female Gambel’s white-crowned sparrows sing in spring and
winter, and the songs are used in an aggressive context. When AVT was
directly infused into the third ventricle of female sparrows, their rate of
singing increased dramatically within five minutes following the treatment
(Maney et al. 1997). The effect of AVT on singing behavior was uniform
across seasons and was not influenced by variations in circulating levels of
gonadal steroids. This observation suggests that AVT is a potent activator
of songs both during breeding and nonbreeding season in females of this
species.
In summary, aggressive vocalizations in birds appear to be largely
dependent on androgen, although the underlying mechanism of vocal activation probably changes between breeding and nonbreeding seasons.
AVT may also be involved in regulating aggressive vocalizations, although
further study is necessary to generalize its role.
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A. Yamaguchi and D.B. Kelley
gambelii), males sing in the winter when circulating levels of T are low
(Logan and Wingfield 1990; Schwabl and Kriner 1991; Wingfield 1994a,
1994b). Castrating wild males does not suppress winter aggression in song
sparrows (Wingfield 1994b). Songs of female cardinals are produced while
plasma levels of testosterone are at undetectable levels (Yamaguchi 1996).
These results indicate that aggressive songs can be independent of androgen. However, a recent study suggests that very low levels of endogenous
androgen and estrogen may in fact be capable of activating aggressive song
production. Inhibition of androgen and estrogen using an aromatase
inhibitor and an antiandrogen during the nonbreeding season, when the T
level is very low, decreased the amount of singing and overall aggression in
male song sparrows (Soma et al. 1999b). Because the gonads do not synthesize steroids during nonbreeding season, nongonadal steroidogenesis
and up-regulation of androgen receptors may account for activation of
aggressive song production during this time (Soma et al. 1999b). In
summary, despite its low circulating levels, androgen may still play a role in
activating aggressive vocalizations in songbirds.
In addition to steroid hormones, AVT may also play a role in activating
aggressive vocalizations in some species of birds. In the field sparrow
Spizellas pusilla, males have two acoustically distinct song types during the
breeding season: a simple, multipurpose song that is used both for courtship
and territorial aggression and a complex song that is used predominantly
in male–male aggression. Goodson (1998) showed that the aggressive songs,
but not the multipurpose songs, of males can be induced by intraventricular infusion of AVT. This finding differs from the role of AVT in frogs, where
AVT selectively activates courtship vocalizations without activating aggressive vocalizations (Semsar et al. 1998).
In previous examples reviewed in this chapter, the effects of AVT are
partly dependent on gonadal steroids. However, in the case of aggressive
vocalizations in songbirds, AVT might activate vocalizations independent
of steroids. Female Gambel’s white-crowned sparrows sing in spring and
winter, and the songs are used in an aggressive context. When AVT was
directly infused into the third ventricle of female sparrows, their rate of
singing increased dramatically within five minutes following the treatment
(Maney et al. 1997). The effect of AVT on singing behavior was uniform
across seasons and was not influenced by variations in circulating levels of
gonadal steroids. This observation suggests that AVT is a potent activator
of songs both during breeding and nonbreeding season in females of this
species.
In summary, aggressive vocalizations in birds appear to be largely
dependent on androgen, although the underlying mechanism of vocal activation probably changes between breeding and nonbreeding seasons.
AVT may also be involved in regulating aggressive vocalizations, although
further study is necessary to generalize its role.
300
A. Yamaguchi and D.B. Kelley