Can testosterone induce the male whistle-shake call in female shelducks Tadorna tadorna?

We investigated by application of testosterone-proprionate whether females of the dimorphic Shelduck Tadorna tadorna are capable of producing vocalisations typical of the male sex. We found that hormone-treated females generated the whistle-shake call, an epigamic vocalization pattern of male Shelducks. Although the female whistle- shake call was male-like in its syntactic form, we observed significant differences in both the phonetic and temporal characteristics of male and female whistle-shake calls. None of the testosterone-treated females was found to produce real male-typical elements. All whistle-shake calls of these females consisted of elements which are also used in calls of untreated females, e.g. the agagak-call. The present endings indicate that the sexual dimorphism in Shelduck vocalisations is not due to early organising actions of sex hormones influencing the differentiation of the brain. It results mainly from adult sex differences in the endogenous testosterone production. Only males produce testosterone in considerable amounts. The inability of testosterone-treated female Shelducks to generate male-typical element might be caused by limitations in the sound-producing organ, the syrinx. Male Shelducks have associated with the syrinx a large resonating chamber, the bulla ossea, which is lacking in females.