Researchers have found a surprising new explanation of how young brains are shaped for sexual behavior later in life.

Immune cells usually ignored by neuroscientists appear to play an important role in determining whether an animal’s sexual behavior will be more typical of a male or female, according to research led by Kathryn Lenz, an assistant professor of psychology and neuroscience at The Ohio State University.

The study, which was done in rats, appears in the Journal of Neuroscience.

To better understand the role of the mast cells in sexual behavior, Lenz and her colleagues silenced the cells in male fetal rats and then observed the rats’ development later in life.

The researchers paired one of these male animals with a female that was receptive to mating and watched to see whether the male sexually pursued the female – basically, whether he chased her and mounted her.

The experimental males were far less interested than typical males, acting almost like females.

The researchers also manipulated female newborn rats, activating the mast cells with a stimulating chemical.

As adults, they acted like males.

“It’s fascinating to watch, because these masculine females don’t have the hardware to engage in male reproductive behavior, but you wouldn’t know it from the way they act,” said Lenz, a researcher in Ohio State’s Institute for Behavioral Medicine Research. “They appear to be strongly motivated to try to engage in male sexual behavior with other females.”

The researchers found that estrogen (which plays a major role in development of masculine traits in rats) activates mast cells in the brain and that those mast cells drive the animal’s sexual development.

Though scientists know that sex differences are programmed by hormones during early development, they have limited information about the cellular-level changes that contribute to the manner in which the brain and behavior are formed.