Data from: Female ornamentation is associated with elevated aggression and testosterone in a tropical songbird

In males, testosterone plays a key role in ornament production and linking
ornamentation with reproductive behaviors and other traits to produce an
integrated phenotype. Less is known about whether females couple
testosterone, ornamentation, and aggressive behaviors to achieve
female-specific combinations of traits. Ornamentation in females may be
the result of correlated expression with male ornamentation, or female
traits could arise as the result of sex specific selection pressures.
Resolving between these alternatives is necessary to understand the degree
to which selection acts on female traits. The White-shouldered Fairywren
(Malurus alboscapulatus) provides a useful context to address these
questions because populations vary in degree of female ornamentation, a
derived trait, whereas male ornamentation is constant across both
populations. We found that ornamented females have higher levels of
circulating testosterone and respond more aggressively to experimental
territorial intrusions than do unornamented females. These findings are
consistent with the idea that, among female White-shouldered Fairywrens,
testosterone may mechanistically link plumage and behavioral traits to
produce an integrated competitive phenotype, as has been reported for
males of closely related species. In contrast, circulating testosterone in
males did not differ significantly between populations. More broadly, our
findings are consistent with ongoing selection on the mechanisms
underlying female ornaments, likely via social selection.