Reproductive behaviour is a motivated behaviour that can be schematically divided into two phases respectively called the “appetitive phase” and the “consummatory phase”, by analogy with the sequences of eating behaviour. This distinction has often been used, both by ethologists and experimental psychologists, for the investigation of mechanisms controlling numerous types of motivated behaviour, such as predation, eating or aggressive behaviour. It is also used by researchers in Jacques Balthazart’s group in their studies on the sexual behaviour of the Japanese quail.

Sexual behaviour is mainly controlled by steroids secreted by the testicles and the ovaries. These controls are however sexually differentiated. It was long thought that male vertebrates developed male-typical behaviour because their testicles produced testosterone. And, according to the same logic, that females adopted female-typical behaviour due to the secretion of an oestrogen, oestradiol, by the ovaries. We now know that testosterone must be transformed into oestradiol in the brain in order to activate male sexual behaviour. At least in quail and numerous other species.

If we administer testosterone to the brain of a female quail, she does not behave like a male. However, if we give oestradiol to a male, he will sometimes behave like a male and sometimes like a female. More eloquent still: when we block the activity of aromatase, an enzyme that catalyses the transformation of testosterone into oestradiol, the effects of the male hormone are suppressed: it is as if the male had been castrated. Therefore, we are entitled to ask ourselves why we do not observe male behaviour in the female, whose ovaries secrete oestradiol in abundance? Quite simply because the brains of the two sexes are anatomically and functionally different! It is therefore not the type of hormone (testosterone or oestradiol) that comes from the testicles or ovaries that conditions the nature of the behaviour, but the type of brain, i.e. male or female.

But beware, this step backwards is only superficial, because the specification of the “sex of the brain” directly and irreversibly depends on the prevalent hormonal context during the embryonic period. In other words, the genes simply control the formation of the testicles or ovaries, which consequently determine the sex of the brain. Jacques Balthazart has shown this in quail: if the brain of a male embryo is experimentally exposed to oestrogens between the ninth and the twelfth day of incubation of the egg, the animal will be born with a female brain. And in the absence of oestrogens, with a male brain.

Today, researchers are capable of manipulating at will the phenotypical, i.e. behavioural, sex of birds, regardless of the genetic sex. Once it has reached adulthood, a male that has received oestradiol during the embryonic stage – in the egg – will behave like a female. And an adult female, like a male, if the production of oestradiol has been blocked at the same stage of development.

In rats and mice, the situation is similar, but it is a mirror image. In other words, it is testosterone that leads the way and not oestradiol. In quail, masculinity is in some way the sexual expression by default, and the presence of female hormones in the egg is required to impose femininity; in rodents, the development of masculinity requires the presence of male hormones during the embryonic period.