Sex Drive in Brain, Not Hormones, Study Suggests

Female mice apparently become as randy as males after their sense of smell gets tampered with, aggressively trying to mount any mouse that moves, research now reveals.

These findings open the question as to whether circuits for male behavior exist dormant in females and vice versa elsewhere—including humans.

In many animals, the vomeronasal organ often helps detect scents. In mice, it especially helps detect [OU1]pheromones, molecules used to attract the opposite sex and to communicate other signals linked with social behavior, such as parenting or aggression. The organ is found in the noses of practically all land animals with a backbone, except humans and other higher primates.

Key to helping signals from this organ travel up neurons to reach the brain is a gene called Trpc2. After they knocked out this gene in mice, Harvard molecular neuroscientist Catherine Dulac and her colleagues found that females displayed uniquely masculine sexual behaviors, such as chasing, mounting and thrusting pelvises against mice, both male and female, as well as giving out ultrasonic mating calls and sniffing derrieres.

"These results are flabbergasting," Dulac said. "Nobody had imagined that a simple mutation like this could induce females to behave so thoroughly like males."

As to whether or not this applies to humans, especially since humans lack vomeronasal organs, "my guess is as good as yours," Dulac told LiveScience. Rodents are driven by smell, while "humans and higher primates are mostly visual—hence pornography!"

The effect if female mutants was also seen when the researchers surgically removed the vomeronasal organs of normal mice. The female rodents who had their sense of smell genetically or surgically short-circuited also abandoned typically maternal behaviors.

For instance, after giving birth, normal female mice spend about 80 percent of their time in their nest nursing their newborns, but the altered females readily wandered away after some two days of motherhood, eventually leaving the nest altogether. While nursing mice ordinarily attack male intruders and reject their sexual advances, the mutant females were docile toward males and seemed highly receptive to their overtures.

Often the differences in behavior seen between the sexes in mammals are attributed to chemicals such as hormones. These new findings suggest otherwise—the researchers found that female mice that had their sense of smell genetically or surgically hampered showed normal estrogen levels, testosterone levels and fertility cycles. Dulac and her colleagues described their findings online August 5 in the journal Nature.

"Our work suggests that neuronal circuits underlying male-specific behaviors develop and persist in the female mouse brain, but are repressed by the normal activity of the vomeronasal organ," Dulac said.

"In fact, our research suggests a new model where exactly the same neural circuitry exists in
males and females ," she added. "You only have to build one brain in a species and that the one brain is built, more or less, the same in the male and the female."

The only thing that differs between the sexes in mice when it comes to sexual behavior is due to how the vomeronasal organ works, Dulac suggested. "While male and female bodies are strikingly different physiologically, it appears the same cannot be said for the brain," she said.

These findings might open up new avenues for investigation into the brain origins of sex-specific human behavior.

Dulac and her colleagues are now studying male mice with genetically disabled vomeronasal organs to see if they display feminine traits.

• Video: Surprising Sex Reversal in Mice
• The Sex Quiz
• Brief History of Sex