My previous post on penis size manifestly struck a chord. And several comments indicated that I should tie up some loose ends, so to speak. So here I aim to clarify size relationships between the flaccid and erect penis, a proposal that the human penis is unusually thick so that it can remove semen of rival males, and an intriguing relationship between finger-length ratios and penis length.

Penis extensibility

One unexpected finding from studies of human penis size is that erect length is not significantly associated with flaccid length. Now this bald statement needs clarification. Clearly, simply plotting erect length against flaccid length yields a positive trend. But there is a covert statistical problem here that I first encountered in studies of the relationship between lifespan and age at first breeding. In 1985, Paul Harvey and Richard Zammuto addressed this topic using data from natural populations of mammals. They reported that “the age at which females first reproduce is strongly correlated with expectations of life at birth” and linked age of sexual maturity to mortality patterns. But, together with William Sutherland and Alan Grafen, Harvey subsequently acknowledged that the statistical correlation was questionable because both variables include the same component. Age at sexual maturity (pre-reproductive lifespan; A) combined with life thereafter (post-reproductive lifespan; B) makes up total lifespan (A+B). It is inevitable that lifespan (A+B) will be positively correlated with age of first breeding (A). But when the data were appropriately re-analysed, cleanly examining pre-reproductive lifespan (A) in relation to post-reproductive lifespan (B), a highly significant correlation remained. This thankfully salvaged a key finding regarding mammalian life histories.

Exactly the same problem arises when examining erect or stretched penis size (A+B) in relation to flaccid penis size (A). A strongly significant positive correlation is inevitably found, as reported in 1996 by Hunter Wessels and colleagues for a sample of 80 men. In this case, however, no significant correlation remains when B (increase through erection) is examined alone in relation to A (flaccid length). This result was confirmed by Roberto Ponchietti and colleagues with a much larger sample of 3,300 young men. Erection typically increases the length of the human penis by about two thirds, but in any individual case the flaccid length of the penis provides no reliable indication of the length that will be achieved when erect.

A significant correlation between erect and flaccid penis lengths (above) disappears when extensibility of the penis is considered (below). Adapted from Wessels et al. (1996)

Uniquely thick

We can forget the oft-repeated myth that human penis length exceeds that in all other primates. Bonobos and probably chimpanzees surpass humans in this respect. (See my January 3, 2015 post: Penis Size Matters.) However, compared to other primates, relative girth (circumference) of the human penis is seemingly unique. In The Naked Ape, Desmond Morris linked this feature to increased pulling and pushing of a woman’s external genitalia, notably the clitoris, during copulation. This proposal incidentally implies that such stimulation must be far greater than in other primates. But in their 1995 book Robin Baker and Mark Bellis proffered a radically different explanation, proposing that the human penis serves as a suction piston to remove semen deposited during prior copulation with a rival male. It should be noted, though, that this interpretation is directly linked to the notion of direct sperm competition between men, which Baker and Bellis have relentlessly promoted. In fact, as indicated in my 2013 book, reliable biological evidence consistently points in the opposite direction: Humans have an array of adaptations for a single-male mating system in contrast to certain primates, such as Rhesus macaques and chimpanzees, which clearly do possess multiple adaptations for sperm competition. (See my posts of August 7, 2013: Dispatch From a Conscientious Objector. And October 16, 2013: Kamikaze Sperms or Flawed Products?) One of the most convincing indicators of adaptation for sperm competition in primates faced with direct sperm competition is a strikingly large sperm midpiece  —  effectively the fuel tank for propulsion. Human sperms have one of the smallest midpieces known among primates.

Gordon Gallup, Rebecca Burch and colleagues subsequently published two papers (2003, 2004) reporting experiments designed to test the Baker/Bellis “plunger” hypothesis that the human penis is adapted to remove semen of rival males. Models of the human penis and vagina were used to assess displacement of prior deposits of semen-like fluid (corn starch solutions) by simulated copulatory movements. Results indicated that marked displacement occurred with the typical human penis shape, especially in association with deep thrusting. Alan Dixson (2012) rightly noted that the conclusions are probably invalid, because the methods used were unrealistic (corn starch?) and open to subjective bias. But the biggest flaw of all is the inherent assumption that over evolutionary time women copulated with multiple partners in rapid succession regularly enough for selection to favour adaptation of the penis for semen removal. Scenario: Man 1 copulates with a woman and minutes afterwards Man 2 whips out his plunger and engages in some post hoc damage limitation. Does this sound remotely likely?

An alternative, more realistic proposal made by Edwin Bowman (2008) is that the greater girth of the human penis may be linked to evolutionary changes in the female pelvis and vagina. He suggests that increase in the diameter of the pelvis to permit birth of a neonate with a larger braincase was associated with widening of the vaginal canal. Accordingly, a thicker penis might have evolved to allow “a satisfactory fit” during copulation.

Finger ratios

Considerable interest has been aroused by findings indicating that the length ratio between the second digit of the hand (index finger; 2D) and the fourth (ring finger; 4D) differs between men and women and reflects the action of testosterone during prenatal life. The index finger is typically shorter than the ring finger in both women and men, but the lengths of the two digits differ more in men, such that they have a smaller value of the 2D:4D ratio. On this basis, In Ho Choi and colleagues examined the relationship between digit ratio and penis length in a 2011 paper. Under anaesthesia, they measured flaccid and stretched penis lengths of 144 Korean men undergoing urological surgery. Data analyses revealed that only body height was significantly correlated with flaccid penis length, while only the 2D:4D ratio was significantly correlated with stretched length. They conclude that the digit ratio can serves as a predictor of adult penis size, which may reflect prenatal effects of testosterone.

Let it be noted that the findings reported by Choi and colleagues suggest a possible link between the digit ratio and stretched length of the penis, not its flaccid length. Combined with evidence regarding the extensibility of the penis, this indicates that studies of women’s responses to flaccid penis length may not be biologically relevant.

References

Baker, R.R. & Bellis, M.A. (1995) Human Sperm Competition: Copulation, Masturbation and Infidelity. London: Chapman & Hall.

Bowman, E.A. (2008) Why the human penis is longer than in the great apes. Archives of Sexual Behavior 37:361.

Choi, I.H., Kim, K.H., Jung, H., Yoon, H.S.J. Kim, S.W. & Tae Beom Kim, T.B. (2011) Second to fourth digit ratio: a predictor of adult penile length. Asian Journal of Andrology 13:710-714.

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

Gallup, G.G., Burch, R.L., Zappieri, M.L., Parvez, R.A., Stockwell, M.L. & Davis, J.A. (2003) The human penis as a semen displacement device. Evolution and Human Behavior 24:277-289.

Gallup, G.G., & Burch, R.L. (2004). Semen displacement as a sperm competition strategy in humans. Evolutionary Psychology 2:12-23.

Harvey, P.H. & Zammuto, R.M. (1985) Patterns of mortality and age at first reproduction in natural populations of mammals. Nature 315:319-320.

Martin, R.D. (2013) How We Do It: The Evolution and Future of Human Reproduction. New York: Basic Books.

Morris, D. (1967) The Naked Ape: A Zoologist's Study of the Human Animal. London: Jonathan Cape.

Ponchietti, R., Mondaini, N., Bonafè, M., Di Loro, F., Biscioni, S. & Masieri, L. (2001) Penile length and circumference: A study on 3,300 young Italian males. European Urology 39:183-186.

Sutherland, W.J., Grafen, A. & Harvey, P.H. (1986) Life history correlations and demography. Nature 320:88.

Wessels, H., Lue, T.F. & McAninch, J.W. (1996) Penile length in the flaccid and erect states: Guidelines for penile augmentation. Journal of Urology 156:995-997.

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