ARE YOU ATTRACTED TO HER, OR TO THE TEEMING COMMUNITY OF MICROORGANISMS LIVING IN HER GUT?
This Valentine’s Day, as you bask in the beauty of your beloved, don’t just thank his or her genes and your good fortune; thank microbes.
Research on the microbes that inhabit our bodies has progressed rapidly in recent years. Scientists think that these communities, most of which live in the gut, shape our health in myriad ways, affecting our vulnerability to allergic diseases like hay fever, how much weight we put on, our susceptibility to infection and maybe even our moods.
They can also, it seems, make us sexy.
Susan Erdman, a microbiologist at MIT, calls it the “glow of health.” The microbes you harbor, she argues, can make your skin smooth and your hair shiny; they may even put a spring in your step. She stumbled on the possibility some years ago when, after feeding mice a probiotic microbe originally isolated from human breast milk, a technician in her lab noticed that the animals grew unusually lustrous fur. Further observation of males revealed thick skin bristling with active follicles, elevated testosterone levels and oversized testicles, which the animals liked showing off.
Microbes had transformed these animals into rodent heartthrobs.
When given to females, the probiotic also prompted deeper changes. Levels of a protein called interleukin 10, which helps to prevent inflammatory disease and ensure the successful pregnancy, went up, as did an important hormone called oxytocin.
Oxytocin, often called the love hormone, helps mammals bond with one another. Our bodies may release it when we kiss (and mean it), when women breastfeed, even when people hang out with good friends. And the elevated oxytocin Erdman saw had important effects during motherhood. Some of the mice in her studies were eating a high-fat, high sugar diet — junk-foody fare that’s known to shift the microbiome into an unhealthy state. Not surprisingly perhaps, mothers that didn’t imbibe the probiotics were less caring and tended to neglect their pups. But mothers that had high oxytocin thanks to the probiotic were nurturing and reared their pups more successfully.
What Erdman’s research suggests is that the microbes we carry, the same ones that make us attractive to potential mates, also directly influence our reproductive success. So when mammals choose mates based on the glow of health, they’re choosing not just an attractive set of genes, but also perhaps a microbial community that might facilitate reproduction.
Another way to look at it: By making their hosts sexy, and by increasing hormones that bring mammals together, microbes help to ensure their own continued existence — the creation of another host. “Everyone wins,” Erdman told me.
Evolutionary biologists have long included microbes and parasites in how they think about sexual reproduction. But the focus has historically been on the deadly kinds. Take sexual reproduction itself. The reason we may even have two sexes, as opposed to just one gender that self-duplicates, is that constantly shuffling our genomes helps us stay ahead of the many parasites and pathogens eager to suck us dry. Sex guarantees the genetic diversity necessary to persevere in a never-ending war, meaning that you can thank disease-causing microbes and parasites for the opportunity to fall in love at all.
And when you kiss your beloved, well, you may have unfriendly microbes to thank there as well. Kissing is nearly ubiquitous in human cultures (although in some it’s more like sniffing). The practice puzzles infectious disease types, because swapping saliva clearly increases the risk of contagion. But maybe that’s the point. Humans carry various chronic viral infections. Acquiring these viruses during pregnancy can harm the fetus. So romantic kissing, some scientists speculate, may allow women to acquire potentially dangerous infections from their babies’ fathers before pregnancy, increasing the odds of healthy gestation. Making out may be a crude form of self-vaccination.
Then there’s the mystery of body odor. Back in the days before deodorant, one’s stench probably conveyed important information. And it still apparently does. In one classic Swiss study, women were asked to sniff T-shirts previously worn by men and rate their pleasantness. The women tended to prefer shirts from men whose immune system genes were most different from their own, and with whom they’d most likely produce the fittest offspring. Their noses led them, unawares, to the best genetic matches.
Here’s the mystery, though. Human sweat doesn’t actually smell. The odor results from microbes feeding on sweat. Armpits are really fermentation crocks emitting what scientists call “volatile organic compounds” and lay people call body odor. So those women may not have been sensing the men directly, but rather the aroma of whatever microbial mix they carried. Whether they’re really smelling and choosing the human genes directly or the microbes, or both, is anyone’s guess.
What about the more brotherly or sisterly type of love — the yearning to be near others of our kind, to not be alone? That, too, may have a microbial component. Animals that congregate in groups, like us, invariably share parasites and other infections. But they may also spread health-promoting microbes. In fact, some surmise that the need to share probiotic microbes could have partly driven the emergence of sociality in animals. Certain salamanders nest in groups to share microbes that protect their eggs against pathogenic fungi, for instance. Some bumble bee colonies share symbiotic microbes that ward off parasites. “People tend to think of diseases, like the flu virus, spreading through social networks,” Elizabeth Archie, a biologist at Notre Dame, told me. “But a lot of the microbes you have are potentially useful. So maybe good things as well as bad things are spreading through the same modes.”
The idea remains unproven in mammals, although people who live together do end up with similar microbiomes, as do baboons that groom one another.
What is clear, however, is that moms often deliberately transmit healthful microbes to their infants. Young elephants eat their mothers’ feces to acquire the microbes needed to digest food. Naked mole rat pups plead for anal excretions from their parents — imparting microbes that also help them thrive. And humans inherit our first large dollop of microbes from our mothers as we pass through the birth canal. Then comes breast milk, which contains special sugars we can’t digest, but which selectively feed certain microbes in our gut. So motherly love and care involves lots of deliberate slathering with a particular microbial culture.
Erdman doesn’t think any of this is accidental. She suspects, in fact, that the mammalian innovations of birthing live young and feeding them milk secreted from what was, millions of years ago, a sweat gland (the proto breast) helped us gain tighter control over the microbes we pass from one generation to the next — to our benefit. And because oxytocin, the “love” hormone unique to mammals, underlies so much of this behavior, and because microbes affect oxytocin levels, Erdman likes to say that “microbes invented mammals.”
So love, desire, the cheesy rom-coms, the sappy ballads, the Shakespearean sonnets — all of them may depend on that teeming ecosystem of microbes within.