In three experiments, we investigated whether
testosterone itself or its metabolites activate
aggression and dominance in white-throated sparrows
Zonotrichia albicollis. Groups of five to
six sparrows, each treated with a different steroid
implanted subcutaneously, were observed in outdoor
aviaries during late winter to determine the birds' rates
of aggression (supplantations and attacks scaled to the
number of available subordinates) and dominance rankings
with opponents not previously encountered.
In Experiment I, testosterone (T) had a greater effect on
aggression and dominance than did androstenedione,
5alpha-dihydrotestosterone (D), androsterone, or
estradiol (E). In Experiment 2, birds with T or D
+ E had higher aggression scores and dominance ranks than
birds with either D or E alone. Birds with T and
D + E did not differ. The testosterone
metobolites, D and E, thus acted
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synergistically to determine rates of aggression and dominance ranks.
To corroborate these results, in Experiment 3 we treated
T-implanted birds with the following blocking agents: ATD,
expected to reduce conversion of T to E (AT birds);
progesterone, expected to reduce conversion of T to D (PT
birds); or both (APT birds). The APT birds had lower
aggression scores and dominance ranks than did AT or PT birds,
despite having higher mean levels of circulating T than AT or PT
birds or birds implanted with T alone. Cyproterone
acetate also reduced aggression socres and dominance in
T-implanted birds.
We conclude that the hormonal control of aggression and
dominance in these birds requires conversion of testosterone to
both androgenic and estrogenic metabolites.
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