IN ALL species that practise sexual reproduction, males and females show gender-specific behaviours. These range from the way they mate to the way they defend—or fail to defend—their territory. Both males and females start out with the same template at birth, but then something acts on the male to masculinise him for life. But nobody knows just how that happens.

It is well known that sex hormones like oestrogen, which is typically seen as a female hormone, and testosterone, similarly seen as a male one, play a role in shaping the neural circuits in the developing brain, and that much of that moulding takes place before birth. It has also been established that testosterone, as well as being a fully functional hormone in its own right, can be (and often is) converted into oestrogen in the body.

Male mice experience a short-lived testosterone surge on the day they are born. It lasts less than 36 hours and the level then remains low until puberty. (In human males, there appears to be a similar neonatal surge.) This pulse of testosterone is believed to be a key event in the masculinisation of the brain. Nirao Shah and his colleagues at the University of California, San Francisco wanted to find out which neurons in the brain were responding to it. What they discovered was a surprise.

It was reasonable to think, as many people did, that androgen receptors—which respond to male hormones—were mediating the manly transformation. But androgen receptors were found to be nearly non-existent in the brains of newborn animals. Dr Shah could not find them earlier in development either, such as just after the fetal testes first started putting out testosterone in a 13-day-old embryo. Without male hormone receptors to respond to testosterone, the researchers started to suspect androgen receptors were not the players they had been assumed to be. Nor, perhaps, was testosterone.

In their study, published this week in Neuron, the researchers decided to look at male mice which had been genetically engineered to lack androgen receptors in their nervous systems. These males still had androgen receptors in their muscles and elsewhere, so they had masculine bodies, and they experienced the testosterone surge and responded to normally circulating testosterone. But their brains were simply not able to detect it.

The mice were compared with normal males in tests of masculinity. In one, a female was put into the cage. Interestingly, the genetically modified mice still showed the classic male-mating repertoire—mounting, penetration and ejaculation. But the researchers noted that they mounted less often, were less apt to penetrate and did not stick at it for as long as the normal mice. Another test turned up similar results. Typically, a strange male entering another male's cage is met with a fight. Again, the mutant mice behaved appropriately, but they were much less aggressive, spent less time fighting and they took longer breaks between attacks. The same was found for scent-marking. Like normal males, the mutant mice urinated in various spots around their cage (unlike females, who create a single latrine). But they deposited significantly fewer urine marks than the normal males.

Because male-typical behaviours developed as the result of a burst of testosterone, but in the absence of receptors for the hormone, the researchers suspect that the testosterone in the surge is being converted into oestrogen to carry out the newborn sexual-differentiation. “Masculinisation of neural pathways in response to the testosterone surge at birth proceeds primarily under the control of oestrogen,” they conclude. Androgen receptors are not the master regulators for male behaviours, but rather, the researchers say, a “gain control mechanism” which amplifies such behaviours—or, when the receptors are absent, dials laddish behaviour down.