Gender bender: One female mutant mouse – with a genetic mutation that affects the function of the vomeronasal organ – acts like a male by attempting mount another.

Credit: Tali Kimchi

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PARIS: The difference between male and female sexual behaviour may be explained, in mice at least, by a tiny organ in the nose rather than gender-specific brain circuitry.

So say investigators in the U.S., who admit to being stunned by the finding and the implications for the understanding of sexuality.

In a study published yesterday in the British journal Nature, the team engineered female lab mice so that the rodents lacked a gene called TRPC2, effectively short-circuiting the so-called vomeronasal organ. This tiny organ in the nose is packed with receptor cells that pick up pheromones – scents that can trigger behaviour such as aggression or sexual responses in many mammals.

Sexually rampant females

To the scientists' surprise, the mutant female mice behaved like sexually rampant males. They sniffed and ran after females, flounced their pelvises, mounted and thrust at male mice, issuing ultrasonic squeaks of the kind that males emit when mating.

"These results are flabbergasting," said Catherine Dulac, a professor of molecular and cellular biology at Harvard University in Cambridge, Massachusetts, who led the work. "Nobody had imagined that a simple [genetic] mutation like this could induce females to behave so thoroughly like males."

The behaviour was not all male, though. The engineered mice mated with males in a female-typical manner. And, unlike normal males, they did not attack other males.
But once their babies were born, they again became like feckless males, insouciant about raising their offspring and keen on having more sex.

Usually, female mice spend around 80 per cent of their time in their nest nursing their newborns and while lactating will attack male intruders.

But, for the mutant mother mice, just two days after giving birth, they started to wander away from the nest and eventually abandoned the babies altogether. When males showed up, they were docile and receptive to courtship.

Blow to brain differences

To check whether there could be any factors in the mutant mice that could induce this dramatic behaviour change, the researchers surgically removed the vomeronasal organs from the noses of normal female mice – and the same thing happened.

The findings may amount to a blow to those who for decades have looked for underlying differences in brain structure to explain why sexual behaviour between males and females is so dissimilar.

"In the big picture, it suggests that the female brain has a perfectly functional male behaviour circuit" which is repressed by signals from the vomeronasal organ, said Dulac.

Seen from the perspective of developmental biology, "the finding is very satisfactory," she added. "It means 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 female."

The results do not apply directly to humans, as we and other higher primates lack a vomeronasal organ. But they may open up new paths of investigation into gender-specific human behaviour, said the researchers.