From hormones to the brain: Of birds and rats

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 Whereas in humans, such interactions are relatively subtle,  in certain male birds,  some of these "physiological" interactions are spectacular.   The most evolved part of the mammalian brain is composed of two large globes called hemispheres.  There is a left hemisphere specialization for the neural control of singing in several species of birds. The brain asymmetries are observed, however, only during the mating season when the bird actually sings.  The hypoglossal nerve, controlling the bird’s syrinx (from which the song emanates), is larger on the left than on the right side of the brain.  It has been elegantly demonstrated that the surge of circulating testosterone occurring at this time is the main cause of the development of the brain asymmetry.   The asymmetry dissipates after the mating season and reappears in the same bird the next year.

Another piece of evidence to the effect that steroid hormones may have something to do with the adult maintenance of gender-specific hemispheric asymmetries comes from research published by a team of Russian researchers. They studied the influence of gonadectomy (surgical removal of the testicles or ovaries) in newborn and mature male and female rats on functional interhemispheric asymmetry of two behaviors:  the reaction of avoidance of pain and investigative activity in the open field.   Potassium was applied to an exposed brain hemisphere of the rat thus temporarily inactivating that hemisphere.   Neonatally gonadectomized rats have no interhemispheric asymmetry of the studied reactions.  In male rats gonadectomized in the mature state, investigative activity in the open field, in contrast to intact animals, was right   hemisphere dominant.  Ovariectomy of mature female rats led to the increase of the dominance of the left  hemisphere in both the emotional and the investigative activity in the open field.  Gonadectomy of male and female mature rats had an opposite effect on the functioning of the right hemisphere: facilitatory in male rats and inhibitory in female ones.