Scientists Ponder an Evolutionary Mystery: The Female Orgasm

An eye is for seeing, a nose is for smelling. Many aspects of the human body have obvious purposes.

But some defy easy explanation. For biologists, few phenomena are as mysterious as the female orgasm.

While orgasms have an important role in a woman’s intimate relationships, the evolutionary roots of the experience — a combination of muscle contractions, hormone release, and intense pleasure — have been difficult to uncover.

For decades, researchers have put forward theories, but none are widely accepted. Now two evolutionary biologists have joined the fray, offering a new way of thinking about the female orgasm based on a reconstruction of its ancient history.

On Monday, in The Journal of Experimental Zoology, the authors conclude that the response originated in mammals more than 150 million years ago as a way to release eggs to be fertilized after sex.

Until now, few scientists have investigated the biology of distantly related animals for clues to the mystery.

“For orgasms, we kept it reserved for humans and primates,” said Mihaela Pavlicev, an evolutionary biologist at University of Cincinnati College of Medicine and an author of the new paper. “We didn’t look to other species to dig deeper and look for the origin.”

The male orgasm has never caused much of a stir among evolutionary biologists. The pleasure is precisely linked to ejaculation, the most important step in passing on a male’s genes to the next generation. That pleasure encourages men to deliver more sperm, which is evolutionarily advantageous.

For women, the evolutionary path is harder to figure out. The muscle contractions that occur during an orgasm are not essential for a woman to become pregnant. And while most men can experience an orgasm during sex, it’s less reliable for women.

In a 2010 survey, 35.6 percent of women said that they hadn’t had an orgasm the most recent time they had sex. Part of the reason for this is anatomy: the clitoris is physically separated from the vagina.

Still, a number of scientists suspect that the female orgasm serves some biological function favored by natural selection. They just need to figure out what it is.

“My gut instinct is that something that matters so much at an emotional level — the intense pleasure of orgasm — would seem to have reproductive consequences,” said David A. Puts, an evolutionary anthropologist at Pennsylvania State University.

Many hypotheses have been put forward. Dr. Puts and his colleagues have carried out studies to test the possibility that orgasms increase the odds that a woman’s eggs are fertilized by a genetically attractive male.

Elisabeth A. Lloyd, a philosopher at Indiana University, isn’t buying it. In 2005, she published a book called “The Case of the Female Orgasm,” in which she reviewed 18 published theories about its function.

None had strong evidence in its favor, she concluded, and many were undermined by other findings about human sexuality. Years of further research have only strengthened her skepticism.

Dr. Lloyd thinks the best explanation for the female orgasm is that it hasn’t served any evolutionary purpose at all. It’s nothing more than the byproduct of the development of the male orgasm. The orgasm is to women, she believes, as nipples are to men.

But now Dr. Pavlicev and her colleague, Günter P. Wagner of Yale University, are making the case that the human female orgasm has a deep evolutionary history that reaches back to early mammals.

They began by getting better acquainted with the sex lives of other animals, poring through obscure old journals to gather information on species ranging from aardvarks to koalas.

They noted that many female mammals release oxytocin and prolactin during sex — the hormones released by women during orgasms. What’s more, in many of those species, females use a radically different kind of reproduction.

While women release an egg each month, other female mammals, such as rabbits and camels, release an egg only after mating with a male.

Ovulatory cycles evolved in only a few lineages of mammals, including our own, Dr. Pavlicev and Dr. Wagner found. Before then, our ancient mammal ancestors originally relied on ovulation triggered by sex with a male.

Those early mammals developed a clitoris inside the vagina. Only in mammals that evolved ovulatory cycles did the clitoris move away. Based on these findings, Dr. Pavlicev and Dr. Wagner argue that the female orgasm first evolved as a reflex to help females become pregnant.

When early mammals mated, the clitoris could send signals to the brain, triggering hormones that released an egg. Once the egg became fertilized, the hormones may have helped ensure it became implanted in the uterus.

This arrangement has worked well for mammals that rarely encounter males. It helps females make the most of each mating.

But eventually some mammals, including primates like us, started spending their lives in social groups. Females had access to regular sex with males, and orgasm as an ovulatory mechanism was no longer so useful. Our female forebears instead evolved a new system: releasing eggs in a regular cycle.

As the original purpose of the orgasm was lost, the clitoris moved away from its original position. Dr. Wagner speculated that this shift was part of evolution’s dismantling of a sensor system: “You don’t want to have the old signal sending noise at the wrong time,” he said.

“Basically, we don’t know why this happened,” he added. But across mammalian species, “it’s just a very strong evolutionary pattern.”

Dr. Lloyd and Dr. Puts welcomed the new study as a provocative addition to the debate over the female orgasms.

“I’m pretty excited that it’s being published,” Dr. Lloyd said, “because people are going to start talking about female orgasms and getting a fresh look at how much we don’t know about female orgasms, and thinking hard about what we need to know.”

The new theory may shed light on how the human female orgasm first evolved, but Dr. Pavlicev and Dr. Wagner said that it doesn’t settle the debate about its current role in women. “All directions are open,” Dr. Wagner said.

Dr. Wagner said that deciphering the history of the female orgasm might improve reproductive medicine.

“I think you’re looking at the whole woman’s reproductive system a little differently when you have a model for how it might have evolved,” he said.