Combined Aromatase Inhibitor and Antiandrogen
Treatment Decreases Territorial Aggression in a Wild
Songbird during the Nonbreeding Season
Kiran K. Soma,*
,1
Kim Sullivan,† and John Wingfield*
*Department of Zoology, University of Washington, Box 351800, Seattle, Washington 98195-1800;
and †Department of Biology, Utah State University, Logan, Utah 84322-5305
Accepted May 27, 1999
Male song sparrows(Melospiza melodia morphna) defend
territories throughout the year in western Washington
State. In the nonbreeding season (autumn and winter),
aggression and song are robustly expressed but plasma
testosterone (T) levels are basal. Also, castration does
not decrease nonbreeding territoriality. In this field
experiment, we asked whether nonbreeding aggression
is independent of T. T can act viaandrogenreceptors or T
can be aromatized to 17-estradiol (E
2
) and act via
estrogen receptors. We treated free-living nonbreeding
birds with an aromatase inhibitor (ATD) and an andro-
gen receptor antagonist (flutamide) in combination. We
then challenged subjects with a live decoy and playback
oftape-recordedsongs.ATD؉flutamide treatmentdecreased
several aggressive behaviors. However, ATD؉flutamide
treatment did not affect body condition, suggesting that
subjects were healthy and that foraging behavior was not
reduced. As expected, ATD؉flutamide treatment in-
creasedplasma T, probablybyblocking negativefeedback on
luteinizing hormone. Surprisingly, ATD؉flutamide treat-
ment increased plasma E
2
. Most other studies using aro-
matase inhibitors have not measured plasma E
2
. How-
ever, it is possible that ATD؉flutamide treatment
decreased local E
2
levels in the brain but not in plasma.
Finally, ATD؉flutamide treatment increased plasma cor-
ticosterone, perhaps in response to increased plasma T
or E
2
. To our knowledge, these are the first data to
suggest that nonbreeding territoriality is regulated by
endogenous steroid hormones. Nongonadal production
of sex steroids may support aggression in the nonbreed-
ingseason.
1999 Academic Press
Key Words: avian; bird; estrogen; sex steroid; territory;
testosterone; winter.
It is well known that many male animals display
territorialaggressionduring the breedingseason,when
plasma testosterone (T) levels are elevated. However,
in some species,males also aggressivelydefendterrito-
ries during the nonbreeding season, when plasma T
levels are low or nondetectable. Territorial behavior
during the nonbreeding season has been documented
in birds, reptiles, and mammals (e.g., Caldwell et al.,
1984; Moore and Marler, 1987; Wingfield et al., 1997).
Very little is known about the regulation of nonbreed-
ing aggression, and it is generally believed to be inde-
pendent of T (e.g., Moore and Marler, 1987; Schwabl
and Kriner, 1991).
The male song sparrowof western Washington State
(Melospiza melodia morphna) displays territorial behav-
ior throughout the year. These birds breed in the
spring and are not migratory (Wingfield and Monk,
1992).After breeding, they do not form large flocks but
instead defend territories throughout the autumn and
winter (Wingfield and Hahn, 1994). Territorial aggres-
sion in the breeding and nonbreeding seasons is
1
To whom correspondence should be addressed. Fax: (206) 543-
3041. E-mail: ksoma@u.washington.edu.
General and Comparative Endocrinology 115, 442–453 (1999)
Article ID gcen.1999.7334, available online at http://www.idealibrary.com on
442
0016-6480/99 $30.00
Copyright
1999 by Academic Press
All rights of reproduction in any form reserved.