Is our ability to differentiate people of another sex innate?

In a laboratory at Stanford University (California, USA), a group of scientists work with mice to look for where some of our fundamental instincts come from or even to question whether those instincts actually exist. On this occasion, Nirao Shah’s team tested rodents’ ability to recognize the sex of another member of their species for the first time, never having encountered animals other than their mother and sisters. Their results indicate that, at least the males, know in a few seconds whether the mouse in front of them shares their sex or not. In their experiments they observed that if it was a female, the male would try to copulate with her and if it was a male he would prepare to fight.

The scientists, who have published their results in the journal Cell, tried to identify if there was any region of the brain of the mice where this behavior was inscribed and if there were anatomical differences between males and females that could explain differences in the way to act.

To begin searching, they focused on brain tissues that respond to sex hormones and produce aromatase, an enzyme that regulates the expression of those hormones. Among those regions, which have a different anatomy and are related to different behaviors depending on whether they are in the brain of a male or female, they focused on one known as the nucleus of the terminal stretch mark bed. In the case of humans, this area is twice the size and more neurons in men than in women. In this region, in mice, one finds a type of neuron that produces aromatase.

To verify the role of these neurons in the recognition of individuals of another sex without the need for learning, scientists first recorded the activity of these cells in the presence of males or females. They then manipulated the neurons to see what was happening. When they were turned off, the mouse lost the ability to recognize the sex of the individual next to it and also the interest in copulating with females or fighting with males. However, scientists observed that when a male was activated neurons imitating what happens when a female appears, he tried to copulate with the individual who approached even if it was a male.

In the case of females, although the same neurons also responded to the appearance of another mouse, there didn’t seem to be an important difference if the animal was a male or a female. Manipulating the activity of neurons also had no effect on the behaviour of mice. According to Nirao Shah, the lead author of the article, “females apparently use another neural system to recognize the sex of other individuals,” but what that is is is still unknown. “We’ll try to find it,” he adds.

Cristina Márquez, a researcher at the Instituto de Neurociencias de Alicante (UMH-CSIC) who also studies neuronal circuits in mice that explain their social behaviour, considers the article very interesting and novel. “Finding neurons that help identify the sexual identity of other animals is very important,” she says. However, he believes that certain interpretations, such as making the leap to making this work in a similar way in humans, are excessive. “We have a much larger cerebral cortex, smell is not that important to us, because we have much less developed smell and we have many more inhibition mechanisms that play an important role in how we interact with other people,” he explains.

In addition, Márquez also argues that stating that this type of result, which ensures that recognition of the opposite sex is innate and always has the same associated reaction, and that it can be applied to humans, can give wrong ideas about how we work. “The fact that something is innate, such as not needing previous experience to recognize an individual of another sex, does not mean that that trait is insensitive to the experience or that it cannot be modified,” he says. In his opinion, it is important not to encourage a dichotomy between what has been learned or what is innate, because “education or culture are not outside our brain; they affect how neurons are activated or what neurotransmitters are produced, it is not outside and inside, it is all the same”.

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