[via telegraph]
The finding signals that not all sex-specific characteristics develop in the womb during pregnancy, as was previously thought.
Scientists came to the discovery by giving baby female rats treatment normally reserved by mother rats for their sons.
Mother rats typically spend more time grooming males. Previous studies have suggested this is necessary for their genitalia to develop properly.
The researchers, from the University of Wisconsin, stroked baby female rats in a similar way. They found that the number of receptors for oestrogen - the female sex hormone - in the stroked rats' brains was lower than in those not stroked, and were of similar levels to that in male rats' brains.
On inspection of the rats' DNA, they found that among stroked females, there were more chemical "caps" on the gene controlling how many oestrogen receptors would be produced.
The scientists believe that as this "capping" is often permanent, the effects of their stroking on sexual characteristics may be long-lasting or even irreversible.
Celia Moore, from the University of Massachusetts, Boston, said the finding should prompt further research into the effects on babies' brains of human mothers treating sons and daughters differently. "Sex may not be just genes and hormones," she told the New Scientist.
If similar trends were detected in humans, it may explain why some medical conditions affect men and women differently. For example, depression is twice as common among women as in men.
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