While recent studies may have shown that there is no such thing as a "male" or "female" brain in humans, in mice there are differences between brains of the sexes. One of these differences relates to the ability of the rodents to smell the scent of females, something that the males can do but the females can’t. Yet while you might expect such differences to be hard wired into the genetics of the animals, it appears that things are a lot more flexible than anyone imagined.
“Obviously, people have different brains. We think differently, we act differently,” explains Timothy E. Holy, senior author of a new study published in Neuron. “There's been a lot of interest over the years in trying to understand what it is that makes one brain different than another brain. In this case, the differences between males and females was due not to innate factors – as you might expect for a pheromone-sensing system – but instead to different sensory experiences.”
The researchers were experimenting using mice, where they found that males contained a specific type of neuron that detected the female’s pheromones, or scent, which triggers the males to start courting and mating behaviors. The smell, in particular, is caused by a chemical known as epitestosterone sulfate, which is thought to be a by-product of the females’ sex hormones, and so it would make sense that only the males could detect it, and that it would be coded for in the DNA of the animals.
Yet for their research, the team from Washington University School of Medicine found something intriguing. While ordinarily the two sexes of mice are kept apart in the lab, they found that when the males were over exposed to the females, it physically altered the specific neurons that detect the female’s pheromones. They found that the males actually lost them, which in turn made them unresponsive to the females scent and lose their interest in the opposite sex. What’s more, when the male mice were then segregated from the females again, after time the neurons actually returned.
By further trying the same experiment with mice that had their testes and ovaries removed, they found the same results. “The neuronal types were not changed in ovariectomized females nor castrated males,” says Pei Sabrina Xu, the second author for the paper. “That tells us it's not a hormonal effect but dominated by sensory experience.”
They think that in the wild this effect would never naturally occur, as males only encounter females occasionally, when they then act with great interest, meaning overexposure to her scent is highly unlikely. What it does show, however, is that the neurons between brains are far more plastic than imagined, and can be physically altered by the environment.
