Original cartoon by Alex Martin
Source: Original cartoon by Alex Martin

The evolutionary significance of penis size has been a topic for abundant speculation, often packaged with the myth that the human phallus is far bigger than in other primates. However, the human penis is actually a little shorter, though much wider, than in bonobos and common chimpanzees. (See my January 3, 2015 post Penis Size Matters and the sequel Expanding on Penis Size of February 4.) Curiously  —  despite the unquestionable need to consider “goodness of fit” (with apologies to statisticians)  —  length and width of the vagina have barely been mentioned.

Size of the human vagina

In a rare discussion of female dimensions, in 2005 Jillian Lloyd and colleagues reported an average vagina length of just under four inches for fifty women, with extremes of two-and-a-half and five inches. Importantly, vagina length did not differ between women with previous births and those without. So the particularly challenging human birth process seemingly causes no lasting distension of the vagina. Yet David Veale and colleagues reported in a very recent survey covering some 15,000 men that the average length of a man’s erect penis is about five-and-a-quarter inches. This is somewhat less than previously reported, but even at that size the average erect penis is a third longer than the average vagina. So it is hardly surprising that women reportedly care more about excessive penis length than men’s preoccupation with bragging rights.

Comparison with non-human primates

Plot by Robert D. Martin of Data from Dixson (2012)

Vagina length plotted against body weight for humans and 27 other primate species. Linked points are for inflated and flat sexual swellings.

Source: Plot by Robert D. Martin of Data from Dixson (2012)

As usual, comparisons with non-human primates place human data in perspective. Alan Dixson’s book Primate Sexuality is once again a prime source, listing vagina lengths for humans and 27 other primate species. The four-and-a-half inches cited for human vagina length (from Bancroft, 1989) is about 10% greater than reported by Jillian Lloyd and colleagues, but still notably less than the length of the average erect penis. Plotting against female body weight, using Dixson’s data, reveals that vagina length scales to body weight with simple proportionality. Despite some scatter, a clear trend is evident and average vagina length for women actually lies close to the best-fit line. So women do not have a particularly long vagina compared to other primates. Strikingly, however, at a little over five inches the vagina of female chimpanzees is distinctly longer than in women. Moreover, across the middle of the menstrual cycle the sex skin in the genital region of female chimpanzees is conspicuously swollen, extending the vagina’s effective length by almost two inches.

Unfortunately, data on vagina width for primates are generally lacking, so it is unknown whether a woman’s vagina is relatively wider than in other primates.

The human clitoris

Anatomically, a woman’s direct counterpart (homologue) of a man’s penis is her clitoris. However, it differs distinctly because the penis has a dual role for urination and insemination. By contrast, a woman’s clitoris is connected solely with copulation and is not even involved in fertilization. The clitoris is a woman’s most sensitive erogenous zone and the main anatomical source of sexual pleasure. And it is isolated from the urinary tract, whose opening (urethra) is more than an inch away. Despite its exclusive link to copulation, the clitoris has been shamefully neglected by investigators. In their 2005 paper, Jillian Lloyd and colleagues baldly commented: “  ………  even some recent text books of anatomy do not include the clitoris on diagrams of the female pelvis.” These authors gave an average of three quarters of an inch for externally measurable clitoris length. But there is extensive variation over an eight-fold range from one fifth of an inch to one-and-a-half inches. Despite its small size, the so-called “love button” contains some eight thousand sensory nerve fibres, double the number in the dome of the penis and surpassing the density anywhere else in the body.

Relabelled illustration drawn by Amphis, from Jesielt / Wikimedia Commons.

Externally visible glans and the large internal parts of the clitoral complex in women.

Source: Relabelled illustration drawn by Amphis, from Jesielt / Wikimedia Commons.

Two recent papers published in 1998 and 2005 by Helen O’Connell and colleagues greatly enhanced our understanding of clitoris anatomy. The first, based on dissection of ten cadavers, revealed that the externally visible clitoris (the glans) is just one small part of a “clitoral complex” that is far more extensive than previously realized. Indeed, a 2012 blog post by Robbie Gonzalez aptly likened the overall complex to a mostly invisible iceberg. The second paper by O’Connell and colleagues used magnetic resonance imaging to study the fine structure of the clitoral system. On each side, the hidden part of the complex consists of a bulb and sponge-like body (corpus cavernosum) extending into a tapering arm (crus). The body and arm together are about 4 inches long, considerably longer than the external glans. The hidden clitoral complex is erectile, whereas this may not be technically true of the glans, although it does become engorged during sexual arousal. The bulbs and bodies together flank the vaginal opening and bulge when erect, compressing it.

In 2010 Odile Buisson used ultrasound scans to investigate the rôle of the clitoris while two volunteer doctors engaged in intercourse. The images revealed that inflation of the vagina by the penis stretched the root of the clitoris, such that it had a very close relationship with the front wall of the vagina, known as the G-spot. The authors concluded from their study: “The clitoris and vagina must be seen as an anatomical and functional unit being activated by vaginal penetration during intercourse.”

A functionless vestige?

In the words of Stephen Jay Gould (1993) “As women have known since the dawn of our time, the primary site for stimulation to orgasm centers upon the clitoris.” And the female orgasm has generally been the main context for discussions of the significance of the clitoris. (See my June 5, 2014 post Female Orgasms: Getting Off or Getting On?). Many proposed explanations boil down to the basic question of whether the clitoris and associated orgasms are adapted for some particular function or merely vestigial by-products. Along with Gould, Elisabeth Lloyd forcefully advocated the notion that a woman’s clitoris, like a man’s nipples, is simply a functionless carry-over from shared early developmental pathways. The main argument underpinning this interpretation is that both occurrence of female orgasms and external clitoris size are so variable that they are seemingly not filtered by natural selection.

In a 2008 paper, Kim Wallen and Elisabeth Lloyd reported that variability in clitoris length is more than threefold greater than for vagina or penis length. However, in subsequent commentaries David Hosken and Vincent Lynch noted two flaws in their argument. First, Hosken emphasized that variation in clitoris size may not tell us anything about female orgasm. Second, size variability does not in fact differ significantly between the clitoris and the penis. In principle, the variability measure used by Wallen and Lloyd  —  the coefficient of variation  —  cancels out differences in average size. However, clitoris length is less than one sixth of penis length, so measurement error has a greater impact. To counteract this problem, Lynch compared variability in clitoris and penis volumes and found no significant difference. In any case, we should hardly expect to achieve meaningful results if we examine the tip of an iceberg instead of the entire thing!

References

Bancroft, J. (1989). Human Sexuality and its Problems (Second Edition). Edinburgh: Churchill Livingstone.

Buisson, O., Foldes, P., Jannini, E. & Mimoun, S. (2010) Coitus as revealed by ultrasound in one volunteer couple. Journal of Sexual Medicine 7:2750-2754.

Di Marino, V. & Lepedi, H. (2014) Anatomic Study of the Clitoris and the Bulbo-Clitoral Organ. Heidelberg: Springer.

Dixson, A.F. (2012) Primate Sexuality: Comparative Studies of the Prosimians, Monkeys, Apes and Human Beings (Second Edition). Oxford: Oxford University Press.

Gonzalez, R. (2012) Blog post for io9, filed to Sexology:

http://io9.com/5876335/until-2009-the-human-clitoris-was-an-absolute-mys...

Hosken, D.J. (2008) Clitoral variation says nothing about female orgasm. Evolution & Development 10:393-395.

Lloyd, E.A. (2005) The Case of the Female Orgasm: Bias in the Science of Evolution. Cambridge, MA: Harvard University Press.

Lloyd, J., Crouch, N.S., Minto, C.L., Liao, L.-M. & Creighton, S.M. (2005) Female genital appearance: ‘normality’ unfolds. British Journal of Obstetrics & Gynaecology 112:643-646.

Lynch, V.J. (2008) Clitoral and penile size variability are not significantly different: lack of evidence for the byproduct theory of the female orgasm. Evolution & Development 10:396-397.

Museum of Sex blog on the internal clitoris:

http://blog.museumofsex.com/the-internal-clitoris/

O'Connell, H.E., Hutson, J.M., Anderson, C.R. & Plenter, R.J. (1998) Anatomical relationship between urethra and clitoris. Journal of Urology 159:1892-1897.

O'Connell, H.E., Sanjeevan, K.V. & Hutson, J.M. (2005) Anatomy of the clitoris. Journal of Urology 174:1189-1195.

Veale, D., Miles, S., Bramley, S., Muir, G. & Hodsoll, J. (2015) Am I normal? A systematic review and construction of nomograms for flaccid and erect penis length and circumference in up to 15 521 men. BJU International doi:10.1111/bju.13010, 1-9.

Verkauf, B.B., Von Thorn, J. & O’Brien, W.F. (1992) Clitoral size in normal women. Obstetrics & Gynecology 80:41-44.

Wallen, K. & Lloyd, E.A. (2008) Clitoral variability compared with penile variability supports nonadaptation of female orgasm. Evolution & Development 10:1-2.

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