The results of the study took the scientists by surprise, as the sex difference they observed turned out to vary with the social status of the birds. The researchers initially found what they expected based on the traditional hypothesis the dominant males had a far larger song nucleus HVC, which was three times the size of that in the females. However, when they compared the subordinate males and females, both of which sing the same duet song, the male HVC was still twice as big as the female HVC. Another brain area, the RA nucleus, which is also responsible for the production of the song, displayed the same pattern. The size differences are mainly due to the higher number of neurons found in these brain areas. Interestingly, no differences were observed in area X, a region of the brain that plays a role in song learning.
An entirely different picture emerged, however, from examination of the gene activity in the HVC. The activity patterns of two genes that code for synapse proteins proved to be far better correlated with the birds' polymorphic song behaviour than the size of their brain areas. When they compared the gene activity patterns in the subordinate males and females, the scientists discovered equal levels of activity for both sexes. However, in dominant females, both genes were markedly more active than in the dominant males.
The findings of the various male-female comparisons do not coincide with the accepted view of the regulation of sex differences in the brain and behaviour. If it is assumed that the greater number of neurons found in the brain of the dominant males is necessary for the singing of the complex solo song, then the sex difference in the
|Contact: Manfred Gahr|