Data from: Androgen-armoured amazons – reversed sex roles in coucals are associated with testosterone in females but not males

In some species, sexual selection is stronger in females than males. In
classically polyandrous birds, for instance, females compete for mating
opportunities and males care for offspring. Sex steroids such as
testosterone have been suggested to regulate the behaviours of
‘role-reversed’ females and males, but comparative studies did not find
evidence for a role of testosterone in relation to sex roles. However, the
large variability of hormone measurements across laboratories may prevent
detecting subtle differences in hormone levels. To circumvent this caveat,
I compared sex steroid concentrations of females and males of two closely
related and cohabiting species with different mating systems, the
classically polyandrous black coucal (Centropus grillii) and the
monogamous white-browed coucal (C. superciliosus). Baseline and
GnRH-induced testosterone concentrations  were twice as high in
female black coucals than  female white-browed coucals, and the
low pre-breeding progesterone concentrations of female black coucals were
consistent with progesterone’s modulatory role during agonistic
interactions in this species. Baseline and GnRH-induced testosterone and
progesterone concentrations did not differ between males of both species.
This study provides first evidence that elevated testosterone is
associated with sex-role reversed traits in females, whereas low levels of
testosterone may not be necessary to facilitate sex-role reversal in
males.