Source: Original cartoon by Alex Martin

My two previous blog posts discussed oviducts—tubes leading from the ovaries to the womb. The first (The Sperm that Fertilizes: Champion Swimmer or Lucky Winner?, posted March 28) tackled a broad topic: the myth of the Great Sperm Race. This phallocentric notion portrays the female reproductive tract as a passive obstacle course in which sperms compete intensely to reach the egg. Evidence that pumping actions in womb and oviducts propel sperms towards the ovaries is ignored. Background research reminded me that the may enhance sperm transport. A 1961 paper by Egli and Michael Newton cited observations on cows indicating the arrival of some sperms in the oviduct less than three minutes after insemination. They attributed this rapid transport to womb contractions stimulated by oxytocin. Now here’s the thing: Oxytocin release also accompanies women’s orgasms, which some authors see as a functionless byproduct (see my blog post Female Orgasms: Getting Off or Getting On? posted June 5, 2015). So does oxytocin fast-track sperm transport by triggering contractions in the womb and oviducts?

Are female orgasms functionless vestiges?

Both sexes have orgasms, but they differ in significant ways. Most importantly, for men orgasm and ejaculation usually coincide, so the function seems self-evident. In women, by contrast, orgasms are notoriously variable. Ten percent of women never experience orgasm during coitus and another 20 percent do so only rarely. Even in women who do have them, only 10 percent of women consistently experience orgasms during coitus. Female orgasms also differ from the male version in that they typically last longer and may occur in rapid succession, often with rising intensity. Just as male orgasm is connected with stimulation of the penis, female orgasm is associated with the clitoris; but women do not ejaculate cells.

Seeking a function for female orgasms has spawned multiple explanations. Crucially, it is often assumed that only human females have orgasms. In his 1967 best-seller The Naked Ape, Desmond Morris suggested a double function: strengthening the pair bond and physically exhausting the woman so that she remains prone after coitus, reducing semen leakage (“poleaxe hypothesis”). Alternative proposed functions include assessment of mate quality and greater efficiency of insemination. More specifically, Robin Baker and Mark Bellis proposed in Human Sperm (1995) that female orgasm has an “upsuck” effect on semen, making conception more likely.

Directly challenging all such explanations, Elisabeth Lloyd’s 2005 monograph The Case of the Female Orgasm systematically opposes the expectation that women’s orgasms serve any specific function. In her view, the orgasm—along with the closely associated clitoris—is an insignificant evolutionary by-product of penis development in men. Absence of any specific biological function could explain why female orgasms vary so greatly and have no obvious link with reproductive success. Echoing the 1992 essay “Male nipples and clitoral ripples” by her mentor Stephen J. Gould, Lloyd proposed a direct parallel with the functionless nipples of men. Predictably, The Case of the Female Orgasm triggered opposition from advocates of evolutionary explanations for orgasm. A balanced and perceptive Nature review by Olivia Judson pointedly bore the title “Anticlimax”.

The Clitoris Unveiled

The most sensitive erogenous zone in women, the clitoris, is the primary trigger of sexual arousal. Unlike a penis, it does not combine reproduction with urination. Its anatomy has been widely neglected and some recent textbooks fail to indicate it in diagrams of female genitals. This to some extent reflects the small size of the externally visible glans, under one inch in length. But in 1998 and 2005 groundbreaking papers by Helen O’Connell and colleagues clearly showed that the glans is but a minor part of an extensive “clitoral complex”, resembling the tip of an iceberg. This substantial, largely concealed complex—unmentioned in Lloyd’s The Case of the Female Orgasm—surely indicates that the clitoris is not a functionless vestige!

Source: Photograph taken by J.P. Brown of a 3D print that he generated using data downloaded from Fab Lab Carrefour Numérique

In 2005 O’Connell and colleagues provided clear anatomical images from magnetic resonance imaging. On either side of the vulva, the submerged part of the complex consists of an internal bulb and an external sponge-like corpus cavernosum extending into a tapering arm (crus). Together, the corpus cavernosum and crus (equivalent to the erectile tissue of the penis) are over four times longer than the externally visible part. Along with the bulbs, they become engorged with blood and compress the vaginal opening during sexual arousal.

Female Orgasms Not Confined to Women

Although it has been widely accepted that female orgasms are unique to human women, Lloyd’s 2005 book cites evidence that at least some nonhuman primates, including chimpanzees and macaques, have orgasms. Yet her 2008 paper co-authored with Kim Wallen unaccountably stated that humans “appear relatively unique among animals in that both males and females can experience orgasm.” However, evidence for responses resembling women’s orgasms has existed for over 50 years. A classic 1968 paper by Doris Zumpe and Richard Michael reported female orgasms during in female Rhesus macaques. Then, in 1980, David Goldfoot and colleagues published results of direct recordings of intense uterine contractions and sharply increased heart rate indicating a in female stumptailed macaques. Soon afterward, a landmark review by Mel Allen and William Lemmon convincingly showed that female orgasms occur in many nonhuman primate species.

Source: Left: Image from Wikimedia Commons. Author: Rolf Dietrich Brecher (2014). File licensed under the Creative Commons Attribution-Share Alike 2.0 Generic license. Right: Adapted from figures in Goldfoot et al. (1980).

Oxytocin and Orgasm

Crucially, it is well established that increased levels of oxytocin accompany female orgasm. Nicknamed the “love hormone”, oxytocin plays several vital rôles in reproduction, including kick-starting and maintaining the birth process and triggering milk ejection during breastfeeding (see my blog post Oxytocin—The Multitasking Love Hormone, May 12, 2015).

Two papers by Marie Carmichael and colleagues published in 1987 and 1994 reported increases in oxytocin levels lasting several minutes with self-stimulated orgasm in both men and women. Circulating levels of oxytocin rise during sexual arousal and significantly exceed baseline during orgasm. Intriguingly, the authors suggested that effects of oxytocin on muscle contraction might facilitate transport of eggs and sperms.

Oxytocin and Sperm Transport

Elevated levels of oxytocin accompanying female orgasms and triggering muscular contractions suggest that orgasms might facilitate sperm transport up the female reproductive tract and thus increase the likelihood of conception.

In a 1998 paper (cited but tacitly ignored by Lloyd), a gynaecological team led by Ludwig Wildt reported striking results from an ingenious study of transportation through a woman’s wombs and oviducts. These researchers deposited sperm-sized albumin spheres labelled with technetium close to the cervix and tracked their migration through the womb and oviducts using X-ray imaging (hysterosalpingography). Remarkably, such inert microspheres are passively transported towards the ovaries and reach the oviducts just minutes after deposition. So the womb evidently acts like a pump. When Wildt and colleagues injected oxytocin, the quantity of spheres transported increased dramatically. Pressure recordings within the womb before and after oxytocin injection revealed increases in both muscle tone and contraction strength.

Source: Adapted from a figure in Kunz et al. 1996

In an excellent 2006 review of scientific evidence relating to female orgasm, Barry Komisaruk and colleagues concluded that it is highly likely that women’s orgasms do, indeed, promote sperm transport through the womb and up the ovaries.

Do orgasms make conception more likely?

A fundamental question regarding the function(s) of orgasm is whether it improves the probability of conception and/or overall reproductive success. Agreeing with Lloyd, Judson noted: “Women can become without orgasm; indeed, some women bear lots of children without ever experiencing one.” However, this oft-repeated truism ignores the possibility that women who experience orgasms might be more likely to conceive and raise a family. Even a relatively small difference in conception frequency or reproductive success would suffice for natural selection to drive the evolution of female orgasm. This possibility has yet to be tested. As Judson rightly concluded: “There are no data showing that orgasm enhances reproductive success; but nor are there data showing that it doesn’t...The sad fact is that, for now, all statements about the evolution of the female orgasm are conjectures in an empirical vacuum.”