Hint: they're for more than just making babies.

The reason for the female orgasm has long eluded scientists. Men need them for reproduction; women don't. So why do female orgasms exist?

Scientists studying this issue are divided, said David Puts, a biological anthropologist at Penn State University. Some scientists think female orgasms are totally purposeless. But evidence suggests that they may have once helped (and perhaps still help) us survive and reproduce.

One theory holds that women have orgasms because men have them, said Kimberly Russell, an ecologist at Rutgers University in New Jersey. Some researchers argue that female orgasms exist because as fetuses, we all start out with the same basic parts, regardless of sex. Orgasms in women, like nipples on men, just happen to stick around.

"It might be an anatomical bonus," she told Live Science. In this scenario, the orgasm didn't evolve specifically for females, and it might not serve a specific evolutionary function for them. 

But there's a problem with the argument that orgasms have no function, said Patricia Brennan, an evolutionary biologist at Mount Holyoke College in Massachusetts. It's not adaptive for our bodies to devote too much energy to traits, like nipples, that aren't beneficial. These traits tend to disappear or become less pronounced over time. That's far from the case for female orgasms, she said. According to the Kinsey Institute, female orgasms tend to last longer than male orgasms and can occur multiple times in a row — something that's rare in men. In other words, female orgasms use a lot of energy for a trait that supposedly has no function, she said.

Plus, there's nothing diminished about the anatomical structures involved in the female orgasm, Brennan noted.

The clitoris, a highly sensitive part of the female genitals that has a key role in orgasms, is homologous to the penis. Like male and female nipples, they grow from the same anatomical structure. But contrary to popular belief, Brennan told Live Science, "a clitoris is not just a mini penis."

The human clitoris has “structures that are incredibly well developed,” Brennan said. "To me, that screams selection."

There are multiple theories about how, exactly, the female orgasm helped our ancestors pass on their genes. Although women don't need to have an orgasm to conceive, some research suggests that wasn't always the case. Many female mammals, including rabbits and cats, ovulate only when they mate. Based on an analysis of how traits have been passed down through the tree of life, one study published in the Journal of Experimental Zoology found that our female ancestors probably needed orgasms in order to reproduce.

But again, this theory doesn't explain why orgasms stuck around in women, Brennan said. 

"If orgasms evolved for some adaptive reason, but they're no longer adaptive, they should have disappeared. And clearly they haven't gone away," Brennan said.

Some research suggests that orgasms still create the perfect conditions for conception — even if they're not necessary to ovulate. One study found that women who had orgasms close to when their male partner did actually "upsucked" more sperm into their bodies compared with women who had orgasms much earlier or later than their partner. Scientists have even tried to draw correlations between the number of orgasms a woman has and the number of children she has. But the evidence for these hypotheses is shaky and doesn't draw a direct causal link between orgasms and conception, Puts told Live Science. 

Plus, these theories leave a major question unanswered, Russell said. What if the orgasm has nothing to do with reproduction? What if, instead, it evolved only for pleasure?

Sex doesn't have to feel good for reproduction to take place, Russell said. "We know this from looking at animals! Sex can be very uncomfortable and still gets done," she said. But culturally, the idea that sex might be for more than just babies is somewhat of a taboo topic, Russell said.

Sex that feels good for both males and females has an important social role, Russell said. It relieves stress and helps partners bond. Ancestral humans might have engaged in sex to create more cohesive groups, smoothing over conflict and cementing their social network. We see these behaviors in other primates, like bonobos, who might use sex to help dispel a fight over a tasty piece of fruit or even a clan rivalry, the BBC reports. It follows from this argument that evolutionarily, female orgasms might have acted as a kind of social glue. 

That pleasure alone is enough to make a trait adaptive goes against popular conceptions of why sex, and orgasms, exist. But for Brennan, it makes perfect sense. "To experience pleasure — that seems evolutionarily like a good idea," she said. 

Originally published on Live Science.