- The female orgasm may have evolved from an ancient reproductive trait
- The hormonal surge women experience during orgasm once stimulated ovulation, research suggests
(CNN)What is the purpose of the ever-elusive female orgasm?
In males, orgasm is invariably required for ejaculation and transfer of sperm, researchers noted in a piece published recently in the Journal of Experimental Zoology. "But in females its function is unclear," wrote Mihaela Pavlicev, a researcher at Cincinnati Children's Hospital Medical Center, and Gunter Wagner, a professor of ecology and evolutionary biology at Yale University.
In women, the orgasm does not contribute to reproductive success and accompanies intercourse only "unreliably," they said. So, what gives?
The researchers examined many species of mammals and proposed that orgasm in women evolved from an ancient trait that once played a key role in reproduction.
To discover the big O's reason for being, the researchers explored its underlying physiology and focused on the hormonal surge that women experience while in the throes of passion. Keeping this foremost in mind, Pavlicev and Wagner then looked at other mammals. Despite the enormous diversity of reproductive styles, they were able to identify core characteristics of reproduction and track how these features evolved.
The evolution of ovulation
Certain species of mammals are reflex ovulators. For example, female cats produce a mature egg ready to be fertilized by sperm only when stimulated by intercourse with a male. By comparison, women are spontaneous ovulators who produce eggs monthly, independent of sexual intercourse.
Looking at the distribution of these two types of ovulation across all the species of mammals, Pavlicev and Wagner inferred that male-induced ovulation must have evolved first and spontaneous ovulation later.
If female orgasm, by causing a hormonal surge, helped induce ovulation, this means it once played a direct role in reproduction. Over time, though, spontaneous ovulation evolved, rendering the female orgasm unnecessary.
Looking at the evolution of female genitals, the researchers found additional evidence that orgasm once played a fundamental role in reproduction. As females moved away from reflex ovulation toward spontaneous ovulation, the clitoris -- the major source of orgasm -- moved away from the location of sexual intercourse, the vagina.
All this adds up to broader implications for women, Pavlicev and Wagner suggested: The low frequency of female orgasm -- only a quarter of all women reliably achieve orgasm during intercourse -- is not an individual woman's failing or a physical impossibility but a natural consequence of evolution.
Others challenged this theory.
Elisabeth Lloyd, faculty scholar at the Kinsey Institute for Research in Sex, Gender and Reproduction, found this new theory to be "an impressive piece of work," yet she also had a "quibble." (To help prepare the paper for publication, Lloyd provided comments for the authors but was not involved in the research.)
The researchers define orgasm completely in terms of hormonal surges, she said, but "there's a lot more to female orgasm than hormonal surges." The standard definition of female orgasm used by scientists includes pelvic muscle contractions, she added.
In her own book published in 2005, "The Case of the Female Orgasm," she examined "all 21 published theories" of female orgasm at that time -- yes, 21 -- and discovered they all basically fell into a couple of categories.
The "byproduct theory" -- the one Lloyd prefers -- is based on the biological fact that males and females develop similar traits in the first two months of life as our basic body plan and tissue patterns get laid down.
"Males get nipples because females need them, and females get orgasms because males need them," she said.
By contrast, the pair-bond theory, which is adaptation-based, understands female orgasm to have evolved for the purpose of strengthening the relationship between a male and a female. In other words, the pleasurable feeling of an orgasm would encourage a woman to return to the man for more.
"Adaptation is a trait that evolves through being selected for its advantages and its contribution to evolutionary fitness," Lloyd explained, adding that any adapted trait would therefore contribute to the number of offspring a person leaves behind.
If female orgasm is an evolutionary adaptation, then you would expect to see more orgasmic women reproducing more children and passing that trait on to their children. But that isn't the case, Lloyd explained, citing one study of more than 8,000 women in which the researchers found no correlation between female orgasm and the number of her offspring. Women who often reach orgasm don't necessarily have more kids than other women.
Could the female orgasm still have a purpose?
"Lack of evidence of an adaptive role for women's orgasm should not be taken as evidence that it is simply a byproduct or 'side effect' of men's orgasm and that it has no role," said Barry Komisaruk, a professor in the psychology department at Rutgers University. Because a woman's orgasm has properties that are unique -- different from a man's orgasm -- it probably has a unique, if unknown, function, he added.
While Komisaruk has spent years investigating the female orgasm, his interest developed from an unlikely start.
"I was studying a reflex called the pseudopregnancy reflex in rats," he explained. As first demonstrated in the 1920s, female rats who mate with a vasectomized male rat will develop a false pregnancy that still includes a change in hormones and even the development of a placenta.
After studying brain cell activity of anesthetized rats, Komisaruk developed a method of studying neurons in awake rats. Here, he discovered the same vaginal stimulation that led to a pseudopregnancy also appeared to decrease pain in female rats.
While this seemed to be the case based on their reaction to a pinched paw, how do you really know whether a rat is feeling less pain during vaginal stimulation? With the help of his former graduate student Beverly Whipple, Komisaruk answered this question by performing experiments with women as subjects and simply asking them.
Komisaruk and Whipple discovered the power of the female orgasm. A woman's pain threshold might increase by 50% -- for some women, by more than 100% -- during orgasm.