What makes a male behave like a male? Many answers may come to mind – societal expectations, culture and environment, and hormones, to name a few. Hardly anyone would guess that estrogen – the female sex hormone – plays a role in male behavior. Yet this is precisely what a team of researchers at the University of California, San Francisco have discovered: estrogen, rather than testosterone, plays a vital role in “masculinizing” the developing brain shortly after birth. Their research appears in the newest issue of Cell.
Shortly after birth, male gonads release a surge of testosterone into the bloodstream. The UCSF research team discovered that the male brain contains a number of neurons equipped with an enzyme called aromatase, which converts the testosterone into estrogen. Once exposed to estrogen, these neurons establish a particular circuitry that is unique to the male brain and is thought to account for stereotypically male behaviors such as aggression and territoriality.
This theory is strengthened by the fact that female mice who were exposed to estrogen shortly after birth become “tomboys,” exhibiting the same aggressiveness and territory-marking behavior as normal male mice. You might reasonably wonder, if estrogen is the female hormone, why don’t all girls end up acting like boys? It turns out that ovaries typically do not secrete any hormone this early in life, which allows the brain to establish female brain circuitry.
Brain development is an extraordinarily complex subject with many unanswered questions. If you would like to learn more about this topic, come to Morehead Planetarium and Science Center to see the Science 360 presentation “The Developing Brain.”
Casey Rawson is the Science Content Developer for Science 360.