Lack of sexual dimorphism in steroid accumulation in vocal control brain regions of duetting song birds.

Tritiated testosterone was injected into bay wrens (Thryothorus nigricapillus), a neotropical species in which the female sings a complex song in intricately timed vocal duets with males. Autoradiographic analysis indicated that male and female wrens have the same proportion of cells labeled by testosterone or its metabolites in two brain regions involved in song: the caudal nucleus of the ventral hyperstriatum (HVc) and the magnocellular nucleus of the anterior neostriatum (MAN). This contrasts with the zebra finch, a species in which only the males sing: a considerably greater proportion of male zebra finch cells in HVc and MAN are labeled than in females. This suggests that female birds that produce complex vocalizations have evolved neural song control systems that are extremely similar to those of males in steroid hormone sensitivity.