Testosterone keeps male brain in shape.

Men are from Mars, and women are from Venus--so claims a best-selling book on male and female behavior. On a more scientific level, neuroscientists indeed have evidence that various brain regions differ in size or activity between men and women. When and why do these gender-based neurological differences arise?

Until recently, scientists pointed to embryonic development, or puberty at the latest, as the period when male and female brains diverge structurally and settle into their final forms. Challenging that view, a new study finds that the presence or absence of androgens--testosterone and related hormones--continues to shape a region of the rat brain well into the animal's adult life.

When adult male rats are castrated, this area of the brain, the medial amygdala, dramatically shrinks in volume. Within a month, it reaches the size seen in a female rat brain, Bradley M. Cooke of the University of California, Berkeley and his colleagues report in the June 22 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES. Conversely, in adult females given testosterone, the medial amygdala balloons to male size within several weeks.

"We were a little surprised to see that the sexual dimorphism could be completely explained by adult circulating androgens," says study coauthor S. Marc Breedlove, also of Berkeley.

The medial amygdala in male rats averages 1.65 times the area's size in female rats, a disparity probably explained by its role in sexual arousal. Nerves from the olfactory system run into the medial amygdala, and male rodents seem to use the region to process information about pheromones and other odor-related cues to reproduction. For example, male rats with damage to the medial amygdala don't become aroused by the scent of a female rat.

Other investigators have demonstrated that altering the amount of androgens in the blood changes the size of certain brain regions. The new study, however, is the first to show in mammals that such manipulation can completely reverse a gender-based structural difference in an adult brain. The medial amygdala's changing volume appears to stem from the nerve cells there swelling or shrinking rather than from the creation of new cells or the death of existing cells, says Breedlove.

The study's findings add to a growing belief that the form and function of the adult brain are not rigidly set. "For a long while, people simply considered structural changes in the adult brain to be impossible," notes Bruce S. McEwen of the Rockefeller University in New York.

McEwen suspects that circulating androgens do more than regulate the size of the medial amygdala. They may also influence the brain region's function by altering the inner workings of nerve cells.

Breedlove agrees, "I wouldn't at all be surprised if there were many other changes that we didn't measure."

One lesson that scientists should draw from this study, he adds, is that the structure of the brain is not set permanently at birth. Consequently, scientists must not regard any differences found between male and female brains, or even between those of heterosexuals and homosexuals, as becoming fixed early in development, says Breedlove.

The study also adds to the list of possible risks faced by athletes and other people using androgen supplements, an issue that gained prominence last year when baseball player Mark McGwire acknowledged taking the testosterone precursor androstenedione. "Steroid hormones aren't just affecting the body. They're also affecting the brain," says Breedlove.