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Health

Girls Gone Boys Gone Wild

Altering a mouse's sense of smell can seriously mess with its gender identity.

By Dr Robert W LashSeptember 11, 2007 5:00 AM

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Conventional wisdom about the sexes—and most movies marketed to men under 24—work from the premise that men’s and women’s brains are just fundamentally different. The medical spin on this has been that testosterone, in addition to providing extra oomph for muscles, is a key component in the wiring of the male brain. The male brain is first exposed to testosterone in utero, changing basic brain biology and imposing masculinity. Then during puberty, another wash of testosterone results in distinctly male thoughts—the fodder for all that guns-girls-and-parties cinema—in the previously testosterone-primed brain. Deprive the brain of testosterone at either time and you risk perturbing the sublime development of maleness.

Well, it turns out that life may not be quite that simple—at least for mice. A new study in Nature suggests that the brain pathways for male sexual behavior are also present in female mice, and with those pathways comes the potential for male behavior. Even more intriguing is that the activation of these pathways appears to be regulated by the animal’s sense of smell. Mice, like many other animals, use chemicals known as pheromones to transmit sexual signals, among others. Investigators have recently found that disabling the part of the nose that receives these signals—the vomeronasal organ (VNO)—produces some pretty strange effects on the way a mouse acts out its sex.

Males without a functioning VNO appear to have trouble distinguishing between male and female. They will try to mate with either one and won’t fight other males. This is certainly strange, but our existing model can live with it. Maybe some pheromone turns on those testosterone-sculpted parts of the male brain. No pheromone, no male behavior.

But when female mice don’t have a functioning VNO, even stranger things begin to happen.Like their male counterparts, these mice will try to mount both male and female mice and they engage in the peculiarly male (and particularly attractive) behaviors of butt sniffing and pelvic thrusting. These female mice will also mate in the more traditional way. However, their maternal instincts are altered: they spend less time with their pups and are less aggressive in defending their nests.

What does this mean for us? Well, human mating is a bit more complicated than it is for mice, and our perfumes have far less power over our brains (whatever the beliefs of pushy department-store perfumers). But it may hint that biological gender differences are a bit less hardwired and more flexible than the conventional wisdom suggests. For now, let’s just inhale deeply and say that these studies raise important questions about how far apart Mars and Venus truly are.

Robert W. Lash, M.D. is an associate professor of internal medicine at the University of Michigan Medical School. His clinical interests include thyroid disease, diabetes, endocrine disorders in pregnancy, osteoporosis and metabolic bone disease, and medical education. A member of the LLuminari team of experts, a board certified internist and endocrinologist, Dr. Lash has an active clinical practice and is a hospitalist at the University of Michigan.

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