Behavior is controlled by the brain, so the brains of male mice must differ from those of female mice —right? Not necessarily, say biologists at Harvard University who have created female mice that exhibit classic male sexual behavior. “Mice have an organ in their nose called the vomeronasal organ, or VNO, that together with the brain detects the pheromones that male and female mice secrete,” Catherine Dulac explains. “These pheromones control mating, aggression and gender identification.” When Dulac and her collaborators disabled the females’ VNO through surgery or genetic mutations, they were surprised to see the mice start behaving like males. “The mutant females were aggressive toward strange males, sniffed at their genitals and mounted them,” Dulac says. The mice remained functionally female, however, and in fact mated and gave birth. Then came the second surprise: the mutant mothers quickly abandoned their nests and young and went off to explore their cages —much as males would. The experiment, Dulac adds, implies that the neuronal circuits for “male” behavior exist in the brains of female mice and that the animals’ VNO, by sensing pheromones, controls which sexual behavior repertoire is expressed.