Between 1 in 500 and 1 in 1000 men have an extra X chromosome, a condition known as Klinefelter's Syndrome. In mice, these extra female genes have an unexpected effect, as the second X chromosome actually makes mice more masculine.

When researchers talk about "masculine" and "feminine" behaviors, they're typically talking about an animal's sexual behavior. And what specific behaviors might those be, pray tell? For the answer, let's go to the new paper by University of Virginia researchers to see just what happens when male mice get a second female sex chromosome:

Counter intuitively, males with two X-chromosomes were faster to ejaculate and display more ejaculations than males with a single X. Moreover, mice of both sexes with two X-chromosomes displayed increased frequencies of mounts and thrusts.


As New Scientist reports, two types of mice were used in the study. One were those with the uncommon but normal XXY chromosomal mutation. The others actually had their Y chromosome shifted to a non-sex chromosome, effectively making them male mice with XX sexual chromosomes. In both cases, the mice had the same testosterone levels as your average XY mouse, but both displayed these more masculine sexual behaviors. This suggests that sexual behaviors are determined by more than just the presence of the hormones testosterone and estrogen.

Since the researchers found the same effect in both the XX and XXY mice, we can feel pretty confident that the result is independent of the Y chromosome. Instead, whatever is making those mice act more aggressively masculine can be traced to their female sex chromosome. It's an open question whether this effect also holds true in humans, but it might well explain why a recent study found that males with Klinefelter's Syndrome tended to have more sex than other guys.

While that second X chromosome is meant to be inactive in both females and any males who happen to have it, this research supports the argument that this isn't the case - up to 25% of this supposedly latent chromosome's genes might actually be expressed. The researchers suspect that the expression of an as yet undiscovered gene is responsible for altering the sexual behavior of these mice, and how this chromosome affects the expression of genes elsewhere might help explain broader behavioral differences between the genders.


Hormones and Behavior via New Scientist. Image by Creations, via Shutterstock.