and Kelley 1986). In leopard frog females, neither gonadectomy nor estrogen with or without progesterone application had any effect on release
calling (Diakow 1978). Thus, it is not clear at present what hormone regulates release calling.
In bullfrogs, males and females produce release calls; during the breeding season, females decrease their rates of release calling, whereas males
increase calling rates (Boyd 1992). Although the overall circulating levels
of androgen and estrogen increase in both sexes during the breeding
season, males have higher levels of androgens, and females have higher
levels of estrogens (Licht et al. 1983; Mendonça et al. 1985). In this species,
AVT plays opposing roles in regulating release calls in males and females,
depending on the concomitant levels of T and E 2 . Injection of AVT in the
spring increases rates of release calling in males, whereas the same treatment decreases calling rates in females (Boyd 1992). Injection of AVT in
the fall has no effect on release calling in either sex (Boyd 1992). AVTinduced suppression of release calling during the breeding season was also
observed in female leopard frogs (Diakow 1978; Diakow and Nemiroff
1981). These findings indicate that AVT combined with high levels of estrogen suppresses release calling, whereas AVT combined with high levels of
testosterone increases release calling rates; neither gonadal steroids nor
AVT alone are sufficient to modify release calling rates. Thus, steroids and
AVT are likely to act synergistically in regulating release calls; steroids may
act as primers for the opposing action of AVT (Emerson and Boyd 1999).
Contrary to these findings, AVT has no effect on release calling rates in
Xenopus females. Instead, endogenous prostaglandin E 2 (PGE 2 ) appears to
play a role in regulating release calling rates in this species. Injection of PGE 2
into intact and ovariectomized females suppressed release calling immediately (within 30 sec to 3 min). Injection of the PGE 2 synthesis inhibitors
indomethacin and flurbiprofen (FBP) blocked chorionic gonadotropininduced suppression of release calling (Weintraub et al. 1985). Thus, which
hormone activates release calling may be species-specific.
3.3. Aggressive Vocalizations
The songs of male birds during the breeding season typically have two functions: courtship and aggression. These courtship/aggressive vocalizations of
male songbirds are androgen-dependent, as we have seen earlier.
In some species of songbirds, however, males sing throughout the year
(e.g., Blanchard and Ericksson 1949). Song during the nonbreeding season
is used solely in aggressive contexts to defend winter territories (e.g.,
Wingfield and Hahn 1994). Females of some species, such as northern cardinals, Cardinalis cardinalis, produce songs with an aggressive function
during the breeding season. Are these aggressive female songs and winter
male songs also androgen-dependent? In mockingbirds (Mimus polyglottos), song sparrows (Melospiza melodidia), European robins (Erithacus
6. Hormonal Control of Communication
299
calling (Diakow 1978). Thus, it is not clear at present what hormone regulates release calling.
In bullfrogs, males and females produce release calls; during the breeding season, females decrease their rates of release calling, whereas males
increase calling rates (Boyd 1992). Although the overall circulating levels
of androgen and estrogen increase in both sexes during the breeding
season, males have higher levels of androgens, and females have higher
levels of estrogens (Licht et al. 1983; Mendonça et al. 1985). In this species,
AVT plays opposing roles in regulating release calls in males and females,
depending on the concomitant levels of T and E 2 . Injection of AVT in the
spring increases rates of release calling in males, whereas the same treatment decreases calling rates in females (Boyd 1992). Injection of AVT in
the fall has no effect on release calling in either sex (Boyd 1992). AVTinduced suppression of release calling during the breeding season was also
observed in female leopard frogs (Diakow 1978; Diakow and Nemiroff
1981). These findings indicate that AVT combined with high levels of estrogen suppresses release calling, whereas AVT combined with high levels of
testosterone increases release calling rates; neither gonadal steroids nor
AVT alone are sufficient to modify release calling rates. Thus, steroids and
AVT are likely to act synergistically in regulating release calls; steroids may
act as primers for the opposing action of AVT (Emerson and Boyd 1999).
Contrary to these findings, AVT has no effect on release calling rates in
Xenopus females. Instead, endogenous prostaglandin E 2 (PGE 2 ) appears to
play a role in regulating release calling rates in this species. Injection of PGE 2
into intact and ovariectomized females suppressed release calling immediately (within 30 sec to 3 min). Injection of the PGE 2 synthesis inhibitors
indomethacin and flurbiprofen (FBP) blocked chorionic gonadotropininduced suppression of release calling (Weintraub et al. 1985). Thus, which
hormone activates release calling may be species-specific.
3.3. Aggressive Vocalizations
The songs of male birds during the breeding season typically have two functions: courtship and aggression. These courtship/aggressive vocalizations of
male songbirds are androgen-dependent, as we have seen earlier.
In some species of songbirds, however, males sing throughout the year
(e.g., Blanchard and Ericksson 1949). Song during the nonbreeding season
is used solely in aggressive contexts to defend winter territories (e.g.,
Wingfield and Hahn 1994). Females of some species, such as northern cardinals, Cardinalis cardinalis, produce songs with an aggressive function
during the breeding season. Are these aggressive female songs and winter
male songs also androgen-dependent? In mockingbirds (Mimus polyglottos), song sparrows (Melospiza melodidia), European robins (Erithacus
6. Hormonal Control of Communication
299