Scientists ponder an evolutionary mystery: The female orgasm

[chahlley]

(Science Today • Friday 5 august 2016)

Scientists ponder an evolutionary mystery: The female orgasm

For orgasms,

we kept it reserved for humans and primates.

We didn’t look to other species to dig deeper and look for the origin.

Professor Mihaela Pavlicev (picture)

an evolutionary biologist at university of Cincinnati College of Medicine and an author of the new paper

NEW YORK — 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.
Earlier this week, 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 fertilised 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 Professor 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.

Still, a number of scientists suspect that the female orgasm serves some biological function favoured 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 Dr David 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 fertilised by a genetically-attractive male.

But Professor Elisabeth Lloyd, a philosopher at Indiana University, is not 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 favour, she concluded, and many were undermined by other findings about human sexuality. Years of further research have only strengthened her scepticism.

Prof Lloyd thinks the best explanation for the female orgasm is that it has not served any evolutionary purpose at all. It is nothing more than the by-product of the development of the male orgasm. The orgasm is to women, she believes, as nipples are to men.

But now, Prof Pavlicev and her colleague, Professor Gunter 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 is 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, the researchers 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, the researchers 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 fertilised, 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. Prof 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.”

Prof Wagner said 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. THE NEW YORK TiMES