According to a new study 'Girl Power' may be key to male aggression.
Experts at the University of California, San Fransisco (UCSF) say that they have identified networks of nerve cells in the brain that are associated with how male mice defend their territory, and found that these cells are controlled by the female hormone oestrogen.
Writing about their findings in the journal Cell, the researchers said that their study suggested a pivotal role for oestrogen, as well as the enzyme aromatase that is responsible for oestrogen synthesis, in male territorial behavior.
"This really changes the way we view male and female behaviors," said Shah, who also is affiliated with the UCSF programs in neuroscience and genetics and who last week received the 2009 Pioneer Award from the National Institutes of Health for his research.
"What we previously looked upon as a single unit of gender-related behavior, we now see as a collection of separate behaviors controlled at least in part by distinct neural pathways," added the senior study author.
Shah pointed out that males and females across all sexually reproducing species display gender-specific behavior in many areas, including mating, territorial marking, aggression and parental care.
Collections of cells form circuits in the brain, referred to as neural pathways, control these and other behaviors.
Shah said that both oestrogen and the male hormone testosterone are known to be essential in developing these circuits and in sex-specific behavior.
However, added the researcher, it has been unclear as to what is the precise role of these hormones, and how they may interact genetically to control these behaviors.
He insisted that his team's work filled in at least one piece of the puzzle by suggesting that the conversion of testosterone in the brain to oestrogen by the enzyme aromatase is critical to developing and activating brain circuits that control male territorial behavior.
Calling the potential conclusions of the study intriguing, Shah said: "We show that exposure to oestrogen neonatally can alter adult sex-specific behaviors in mice."
However, those findings did not appear to apply to sexual behavior.
While male mice reliably mate frequently with females, the oestrogen-treated females showed no difference from untreated females when exposed to normal females that were sexually receptive, or in heat.
But the estrogen-treated females did not display typical female sexual behavior: they mated much less frequently with males and even attacked and chased them.
The researchers proposed that these aromatase-expressing regions of the brain could form an interconnected network that regulates sex-specific behaviors.
Shah, however, insisted that more studies on the role of the development of sex-specific neural pathways were required, and hat many additional factors, including genetics and socialization, could contribute to sexual differentiation.