Clues From Transsexual Rats

In at least one animal, the laboratory rat, nature seems much more important than nurture in determining sexual orientation and behavior. At the University of California, Los Angeles, neuroendocrinologis t Roger Gorski is learning exactly what little boy rats are made of.

First of all, they need testosterone and plenty of it early in life. Gorski and his team have found that if they castrate rats just after birth, the animals will exhibit behavior typical of a she-rat with the hots: arching their backs, flexing their tails and allowing other males to mount them. But by injecting these neutered males with testosterone, researchers can return them to maleness. However, such "rescues" work only during the first five days after birth. At day six, the castrates are permanent transsexuals. "If these rats could talk," Gorski speculates, "I think they might say, 'I'm a female. Get me out of this male's body.' "

! Even more intriguing, the UCLA researcher has learned that sex hormones (or the lack thereof) affect the anatomy of a rat's brain. Buried deep beneath the cerebral folds, Gorski discovered a part of the brain that appears to be involved in regulating sexual behavior and is five times as large in males as in females. But without testosterone this specialized region shrinks in castrated subjects. "In rats, sexual behavior is totally dependent on hormones," concludes Gorski. In humans, he allows, things are not nearly so simple.