This Is Your Brain on Sex (11 page)

Our Primates, Ourselves

(or Why We Are Not Slaves to Our Hormones)

A few months ago a friend showed
me an ebook she had downloaded to help explain the innumerable changes her daughter might experience during puberty. It was a modern-day version of the old-fashioned
You’re Growing Up!
pamphlets we received in our own preadolescence, often at the doctor’s office or in health class, filled with illustrations of pink fallopian tubes and pubic hair growth. As I looked through it, I was struck by the content. In one chapter the author wrote, “Your brain guides all of these changes by using chemical messengers called hormones. Hormones affect many different parts of your body. . . . These chemicals also work on your thoughts and emotions and will affect everything you say and do.”
¹

Everything you say and do.
From our earliest years we are told that hormones can influence everything, from our boobs (or balls, as the case may be) to our brains to our behaviors
.
Hormones will flood our system. They will take over and rage out of control. And that impact goes beyond your teenager’s extreme behavioral ups and downs. We talk about hormones to explain why a two-year-old boy likes to roughhouse, why our adult moods swing high and low, and why a friend markedly ogles a member of the opposite sex. Our hormones, they say, are a motivating force behind our desire to seek out love and consummate it. But the key word here is
motivate
.

The two hormones that help us along the journey from childhood to adulthood are also implicated in love and sexual behaviors: testosterone and estrogen. The two are often referred to as if they were sex-specific—testosterone for the boys and estrogen for
the girls—but as with oxytocin and vasopressin, it is not quite that simple. Both sexes have ample quantities of each in the body, as well as several receptor types for each across the brain. They work both together and apart to help us grow and mate.

Donatella Marazziti’s work demonstrated that there were no significant changes in estrogen or progesterone in people who fell in love. But she did find changes in testosterone levels.
²
It is possible that sex hormones are true to their name and are more involved in mediating sexual behaviors than in the gushy love stuff.

That’s right—
mediate
. Our behaviors are not regulated by our hormones. This becomes clear when you see an animal truly in thrall to estrogen and testosterone, like a common rat. Hormones actually control sexual behavior in this species—not mediate or motivate, but control. The female rat ovulation cycle lasts four to five days. When the female is at her most fertile, her hormone levels rise and her back arches up, exposing her private parts to the world. This reflex is called lordosis. It is a sign to all the boy rats that the girl is ready to go (not to mention that lordosis makes it a heck of a lot easier for the male to mount on up to get the deed done). The female rat does not have to consider whether she feels up to sex after a long day of running mazes in the lab. There is no worry about emotional readiness or whether she looks fat. Her hormone levels let her know it is time to get busy. So she does. Otherwise she cannot be bothered. It’s just that simple.

Males have it even easier. They try to mate any fertile female they smell. Provided there is no other male around, lordosis means our young buck is not going to have much trouble. All he needs is right there, open and waiting for him. In short, hormones regulate what amounts to a sexual transaction in rats. And by doing so, hormones ensure that rats are prodigious breeders.

You can see similar hormonal effects, though not quite as regulatory, in other species too. Female dogs go into heat. Female baboons’ bottoms swell and turn red to let the males know they are fertile and ready. In males high levels of testosterone give certain bird species brighter plumage and prettier singing voices to woo the females. These are outward signs of what is happening inside the body and the brain, making it easy for the animals to know when they will get
lucky. There are no games to be played when your body openly betrays your hormonal state.

It is not as straightforward, however, in people. Our sexual behavior over the past hundred thousand years has been emancipated from our hormones. Women acquiesce to nookie not only during the fertile days of their cycle, a phase referred to as estrus; we are often quite willing no matter what time of the month it is. And men do not seem to have any issues mounting us even when we aren’t ovulating. Don’t get me wrong: estrogen, testosterone, and other hormones are still quite necessary for reproduction. Any couple going through fertility treatments can tell you that. But they are not deciding when and where we are having sex.

“Hormones are not absolute regulators of behavior,” said Kim Wallen, a neuroendocrinologist at Emory University’s Yerkes National Primate Research Center. “The function of hormones is to shift the balance of behavior in one direction or another. The presence of certain hormones doesn’t mean you will exhibit a certain behavior but rather increases the probability that you might.”

That increased probability manifests itself in interesting ways. Though you may associate ovulation with bloat and moodiness, hallmark symptoms of premenstrual syndrome, research suggests that it can make a woman feel quite sexy. Kristina Durante, a social psychologist at the University of Minnesota, has discovered that ovulation is linked to women’s buying more revealing clothing, showing heightened interest in manly men, and unconsciously heading out to the clubs. Once they get to that nightclub, ovulating women are more receptive to male attention. Nicolas Guéguen, a researcher at Université de Bretagne-Sud in France, found that women at the height of their cycle are more likely to accept a dance invitation from a stranger.
³
If they happen to work at a club, say as exotic dancers, they will pull in higher tips after lap dances when they are most fertile.
⁴
Men consistently rate ovulating women as having more attractive body scent, facial features, and body symmetry. And several studies have now reported that women with high levels of estradiol, a form of estrogen, are more likely to cheat on committed partners.

Men too are influenced by hormones. Though studies of castrated males have shown that lack of testes (and therefore lack of testosterone) does not always stop the ability to have sex or reach
orgasm, more or less testosterone has been linked to changes in aggressive behaviors, risk taking, energy levels, and libido.

In humans it is clear that sex hormones work in subtle ways, so subtle that some evolutionary biologists have argued that human females have a “hidden estrus.” But hidden or not (and given the results of some of these studies, it doesn’t seem quite so hidden), the sex hormones are still influencing behavior—on both the giving and receiving sides. Scientists believe that hormones may be doing this by direct action on the brain.

Receiving Hormones in the Brain

Sex hormones are not just racing around in your bloodstream without direction. For some time scientists have known that there are receptors for these hormones all over the body. While in the library at the Kinsey Institute for Research in Sex, Gender, and Reproduction, I perused the unpublished works of John Money, a pioneering sex researcher at Johns Hopkins University in the 1950s.
⁵
In one of his unpublished works about sexuality and gender he lamented that we still did not understand the full reach of hormones in eroticism or sexual behavior. His hope was that the future would bring enlightenment. And it has, somewhat. We now know that the brain is full of receptors for sex steroids, and that these hormones act on brain cells both as a mediator—helping other chemicals do their work in the synapse—and through direct action on their own. They can act as neurotransmitters in their own right, and are both manufactured in and working their signaling magic in the brain.
⁶
But what exactly are they doing? We haven’t figured it all out yet.

“It’s outrageously complicated,” said Paul Micevych, a molecular biologist who studies estradiol signaling in the brain at the University of California, Los Angeles. “There isn’t just one way of signaling by estrogen.” The same is true for testosterone. Several receptor types have been identified for both sex hormones; however, there are probably several more that have yet to be discovered—and this is not the only complicating factor.

Like oxytocin and vasopressin, estrogen and testosterone are very similar compounds
.
Very
similar. In fact throw a little aromatase, a type of enzyme, on an androgen like testosterone and it will change the chemical structure of the molecule—into estrogen. The male brain has a lot of aromatase, and for this reason all that testosterone in the blood may not reach the brain in the same state. Nor is it involved in only reproductive behaviors.
⁷
“A lot of the activities that we think of being androgenic are really, at the end, estrogenic,” said Micevych. “That is, the androgen testosterone is converted to estradiol by aromatase. Estradiol then binds to a receptor in the neuronal circuits that affect behavior. It’s not one over the other, it’s both.”

This can be found in aggression. Many studies have linked high levels of testosterone to aggressive behaviors. We think of it as a purely androgenic phenomenon. But guess what? If you knock out estrogen receptors in the mouse brain, overall aggressive behavior decreases.

So what else do we know about estrogen signaling in the brain? To date, two estrogen receptors have been definitively identified, estrogen receptor alpha and estrogen receptor beta, though there is a third suspect and probably a few others that have yet to be discovered. It is the alpha version, in the hypothalamus, that appears to be directly involved with reproductive behaviors. But without our knowing all the receptor types, true understanding of the molecule’s effects remains elusive. Still, in the two existing receptors, it would seem estrogen helps the brain process information. “Estrogens work in the brain like they are opening a gate,” said Micevych. “The same chemical and environmental cues are there whether the estrogen is or not. But when a brain is exposed to estrogen, it opens that gate, so to speak, and allows the information to flow to the right spots to help influence a particular behavior.”

So perhaps estrogen makes us more likely to subconsciously pick up social cues. It may enhance the sound of an attractive man’s voice or enhance the feeling of a woman’s touch. Yet Micevych cautions that there is still a lot to learn at the molecular level before we can even think about translating hormone signaling in the brain to the behavioral level in a mechanistic manner. It’s a very tricky thing to study, due to those multiple signaling pathways, those uncharacterized receptor types, and the transformation of androgens to estrogens. And that is all before you add in the cross-talk with other neuroactive messengers. Because, wouldn’t you know it, estrogen and our old friend oxytocin
interact in areas of the brain like the hypothalamus.
⁸

Micevych, a tall man with a striking, angular face and gravelly voice, was happy to discuss these matters further when I caught up with him at a neuroscience conference. “It’s not even direct cross-talk—the two somehow activate a third type of receptor, called the metabotropic glutamate receptor, in the hypothalamus, the area of the brain that regulates sexual behavior,” he said. “Both the estrogen receptor and the oxytocin receptor compete to activate this third receptor. Both estradiol and oxytocin lead to activation of the metabotropic glutamate receptor, and the response is the same whether estrogen activates that receptor first, before the oxytocin, or vice versa. Interestingly, there is no augmentation of the signaling if they are both applied together. It appears to be an internal check on oxytocin signaling by estradiol and estradiol signaling by oxytocin.”

Dr. Money might have hoped for more concrete answers by now about the exact role hormones play in sexual behavior. But it would seem research has uncovered as many new questions as answers. And remember, it is not just biology; the environment plays a role too. Sexual behavior, and thus the activity of these hormones, is context-dependent. Environment matters—and it matters a lot.

“Older studies show that if younger men play a game of football and win, their testosterone levels go up,” said Julia Heiman, director of the Kinsey Institute. “It’s not so dissimilar to what happens when a new monkey of low status is being introduced to a group. As they gain status in the group, their testosterone levels also rise.” This brings up a good point. Since it can be difficult to look at the influence of hormones on human sexual behavior, with or without context, how can researchers learn more about how context and hormones interact to result in different behaviors? As it turns out, there is a fair amount to be learned just by watching monkeys going at it.

Hot Monkey Loving

As I said, studying sexual behavior can be a tricky thing. I know I’m repeating myself, but it’s an important point. Human beings are not so keen on inviting investigators into their
bedrooms, and university ethics boards frown on people having sex in academic laboratories. Maybe we are intimidated by the idea of lab coats and fervent scribbling, or maybe it’s the idea that our sex lives are no one else’s business—but despite the fact that sex is openly discussed in many of our favorite magazines and television shows, we are still not all that comfortable talking about it ourselves, let alone examining the specifics. Certainly not in an unbiased scientific fashion, at least. While my girlfriends and I have dissected a few sexual acts over cocktails, I could not tell you with any accuracy how many sexual partners each has had, or which acts, for them individually, constitute “sex.” Even as sex becomes a less taboo topic for discussion, we still seem to be holding back. After all, we were raised to believe sex was private, if not outright naughty, and it can be hard to break away from those notions.