Love, Passion, and Science: A Perfect Match

Animal models may not be the most romantic way to study love, but you have to start somewhere—and proposing double-blind trials of human mating would never pass an ethics committee (though it might make for an interesting reality show). So, animals it is.

In 2011, Sharon and colleagues at Tel Aviv University published a study linking digestion to the heart. At least in fruit flies, mate choice seems to be influenced by gut bacteria.

Researchers divided Drosophila melanogaster—the common fruit fly used in countless biology experiments—into two groups. One group was fed a sugar-based diet, the other starch. Since diet shapes microbiota (the community of microbes in the gut), the two groups developed completely different microbial profiles over the course of a year.

When the researchers tested mating preferences, they found flies chose partners based on diet. Sugar-fed flies preferred sugar-fed partners. Starch-fed flies preferred starch-fed partners. But when given antibiotics, the preference vanished—suggesting the selectivity was driven by gut microbes. Reintroducing the original microbiota restored the sexual preference.

It was one of the first studies suggesting a possible correlation between microbiota and sexual attraction—in flies. The researchers proposed that the effect might be mediated by pheromones. Some labs have replicated the findings, others not. Either way, it raises an intriguing question: could gut bacteria play a role in sexual attraction?

Mammals

Of course, results in animals don’t automatically translate to humans. That said, a 2016 paper in Microbiome and Behaviour hinted that bacteria might make mammals—including us—more attractive.

When mice were fed a probiotic isolated from human breast milk, males grew shinier fur, had higher testosterone levels, and even larger testicles—something they proudly displayed. Females given the probiotic released more oxytocin.

Nicknamed the “love hormone,” oxytocin boosts social bonding and also strengthens maternal behavior and breastfeeding. Germ-free mice—raised without microbes—tend to be antisocial, a trait that disappears when their guts are colonized by bacteria.

So microbes may do more than just make us look good: they might also ensure reproductive success by strengthening bonds between partners and between mothers and offspring. The advantage for the bacteria? More hosts for future generations.

A passionate kiss also triggers oxytocin release. And guess what else happens when we kiss? We exchange bacteria. This may be beneficial: by swapping microbes, partners expose each other to new organisms, potentially broadening immune defenses in their future children. In other words, there’s an evolutionary reason why kissing feels so good. Nature is nothing if not romantic. Humans, Finally

In 1995, Claus Wedekind conducted his now-famous “sweaty T-shirt experiment.” Men wore the same shirt for two days, and women were asked to pick their preferred partners based on scent. The women consistently preferred men whose genetic profiles differed from their own.

A decade later, a Brazilian research team replicated the experiment and found the same result. Since sweat itself is odorless, the smell that guided the women’s choices came from bacteria. Once again, microbes were influencing mate selection.

But microbes aren’t the only factor. Other biological cues shape attraction. For example, studies show that heterosexual women often prefer bearded men. Why? The same reason peacocks have colorful tails. Neither beards nor peacock tails are particularly useful—but both signal high testosterone and, historically, stronger, more aggressive males who could defend mates and offspring. Among peacocks, males with larger tails even produce larger chicks. Dancing Days

Of course, impressive traits only go so far. A flashy tail—or a well-groomed beard—means little if the male can’t dance.

Many bird species perform elaborate courtship rituals. Birds-of-paradise, for instance, showcase their best moves, often practicing in groups from a young age. The females, meanwhile, do the hard work of building nests and raising chicks while the males rehearse their choreography.

And what does all this mean for readers hoping to find a partner? Honestly, not much. The microbiome studies are new and inconclusive, and humans are not flies, mice, or birds.

That said—a neatly trimmed beard and a little ballroom dancing never hurt anyone. ✍️ Natalia Pasternak is a researcher at the Institute of Biomedical Sciences at the University of São Paulo (USP), national coordinator of Pint of Science Brazil, and president of the Instituto Questão de Ciência.