Abstract
Sexually dimorphic anatomy of brain areas is thought to be causally linked to sex differences in behaviour and cognitive functions. The sex with the regional size advantage (male or female) differs between brain areas and species. Among adult songbirds, males have larger brain areas such as the HVC (proper name) and RA (robust nucleus of the arcopallium) that control the production of learned songs. Forest weavers (Ploceus bicolor) mated pairs sing a unison duet in which male and female mates learn to produce identical songs. We show with histological techniques that the volume and neuron numbers of HVC and RA were ≥1.5 times larger in males than in females despite their identical songs. In contrast, using in-situ hybridizations, females have much higher (30–70%) expression levels of mRNA of a number of synapse-related proteins in HVC and/or RA than their male counterparts. Male-typical and female-typical sexual differentiation appears to act on different aspects of the phenotypes within the same brain areas, leading females and males to produce the same behaviour using different cellular mechanisms.
Highlights
Sex differences in behaviour, in particular in the realm of reproduction, are common in all vertebrates
We compared the neuroanatomy of vocal control areas in terms of area size and neuron numbers between male and female mates that were observed to defend their territory with dueting in their natural habitat
There are local song dialects that differ in the number of song syllables [20], pair members in all areas studied have an identical song, which is mostly uttered in unison [16,17]
Summary
In particular in the realm of reproduction, are common in all vertebrates. Activity patterns of the HVC and RA appear to be uniquely associated with song syllable and song element identity, respectively [8,9] These areas seem to differ between males and females in size and neuron numbers in those songbird species in which males and females differ in their vocal behaviour [10,11,12]. The sex with smaller neuron numbers might have more complex network properties, as suggested for the human cortex [3] To this end we report here on a dueting songbird species, the forest weaver (Ploceus bicolor), in which male and female mates sing in unison; they learn to sing an identical song during pair formation [16,17]. Sex differences in gene expression of birds are not reported to be regulated by gene dosage compensation [18] and should be higher in males since female birds are the heterogametic sex
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