The Human Penis

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It is unfortunate that the Human Penis has been subject to so many incorrect theories as to how its specific morphology has been influenced by evolution. Most of these incorrect assertions about the evolutionary history of the Human Penis have been influenced by a 1984 paper by Robert L. Smith, having described the human penis as "extraordinary relative to the other hominoids".[1] This view has been further advanced by the likes of Robin Baker and Mark Bellis (The pioneers for the idea that Sperm Competition has played a major role influencing Human Evolution) whom have said that the human penis "is nearly twice as long and over twice as wide as that of the chimpanzee". Alison Jolly has also made similar remarks in her book[2] saying that the "‘peculiarity of humans is that the penis is twice the size for body weight as that of any other primate." In 2000 Geoffrey Miller published a book where he wrote that "adult human males have the longest, thickest and most flexible penises of any living primate".[3] More recent example can be seen in the popular book Sex at Dawn (a book that can only be described as the largest collection of pseudoscientific claims on human sexuality to have ever existed) by Christopher Ryan and Cacilda Jetha describing humans as the "Great Ape with the great penis".[4] It should be interesting to note the both Miller and Ryan are both evolutionary psychologists, not biologists, as well as polyamorists. Typically to account for the apparently grandiose status of the human penis in comparison to other primates, one of the the four following theories are offered as an explanation:

  1. The Length of the Human Penis has evolved to deliver sperm as closely as possible to the Cervix so as to gain an advantage during sperm competition. (Smith 1984[1])
  2. The size and shape of the human penis has evolved to displace rival sperm and thus gain an advantage during sperm competition. (Baker and Bellis 1995[5]; Gallup et al. 2003[6]; Gallup and Burch 2004[7])
  3. The size of the penis has evolved to promote female orgasm; as female orgasm influences sperm retention and transport, larger penises will be selected for in order to gain an advantage during sperm competition. (Fisher 1983[8]; Fisher 1992[9]; Small 1995[10]; Baker and Bellis 1993[11][12] & 1995[13]; Miller 2000[3])
  4. The human penis is more visually accessible than other primate penises and thus has evolved via sexual selection. (Short 1980[14]; Diamond 1997[15])

Penis Size[edit | edit source]

Keeping in mind that the average human penis is between 13-15cm in length and 10-12cm in circumference when erect, let's see what other primates look like in comparison:

  • Pan Troglodyte
    • Erect Length 14.4cm (Dixson and Mundy 1994)[16]
  • Pan Paniscus
    • Stretched length 17cm (Dahl 1988)[17]
  • Gorilla Gorilla
    • Erect Length 6.5cm (Short 1980)[14]
  • Pongo spp.
    • Erect Length 8.75 (Dahl 1988)[17]

As we can see, the human penis is not as magnificent as some would make it out to be. It is similar to the chimpanzee in erect length, only slightly smaller and has a shorter penis that the bonobo (Stretched length and erect length are approximately the same length). It should also be noted that all the above measurements are greater than what was originally claimed by Smith 1984[1] whom was no doubt the single biggest influence on the view that sperm competition played a major role in human evolution (this is especially true in the field of evolutionary psychology unfortunately). Smith reported measurements of 8cm for the chimpanzee, 4cm for the orang-utan and 3cm for the gorilla. Primatologist Alan Dixson has said in his book about human mating systems that the human penis is not " exceptionally long in relation to those of other primate species, especially when their body sizes are taken into account."[18]

A more likely mechanical explanation for the evolution of the human penis, that does not require the invoking of sperm competition, can be found in a 2008 paper by Edwin Bowman.[19] It posits that the size of the human penis has likely co-evolved with the increasing size and width of the female pelvis, due to the increasingly larger heads of new-borns, in order to have a more satisfactory 'fit'. Regardless of whether or not sperm competition is present, there is always a selection pressure for males to efficiently place sperm so as to maximally increase the odds of fertilization.

The female vagina is between 15-18cm in length when aroused. Let's say a 5cm penis ejaculates a further 4cm upon orgasm; that males sperm still has to travel an additional 6-9cm before even reaching the uterus; this will obviously incur heavy loses amongst his sperm and thus lower the chance of pregnancy. It a man with an average penis with 14cm ejaculated up to 4cm inside the vagina, his sperm do not need to travel very far in order to reach the uterus. Thus there is a selective pressure for penises between the range of 13-15cm. Genital co-evolution is not unique to humans and can be found in mammals outside of primates.

Sperm Competition[edit | edit source]

It is useful to examine how sperm competition has influenced the morphology of the penis in other primates where sperm competition in known to have existed. What we find in these multi-male/multi-female mating systems is that they have larger testes and have much more complex penis morphologies than what is typically found in polygynous and monogamous species.[20][21][22][23] Penises also have a tendency to be longer (the length of the human penis can be explained via genital co-evolution and does not require the invocation of sperm competition) and more distally complex. Considering that the Human penis is unapologetically simple is evidence against sperm competition having influenced human penis morphology. To once again refer to Alan Dixson and his book on human mating systems, he compared and rated the penile complexity (length, distal complexity, size of baculum and penis spines) of roughly 48 primate genera.[18] Multi-male/multi-female mating systems scored higher in penile length, distal complexity, baculum length and penile spines than polygynous or monogamous mating systems.

Human penis size was, overall, not exceptional when compared to prosimians, monkeys and apes; scoring highly only in penile length (which can be explained via genital co-evolution). Humans scored similarly to a number of monogamous and polygynous species like Leontopithecus, Callimico, Erythrocebus and Theropithecus. Distal complexity (the shape and size of the glans or at least its equivalent) was found to be as complex as 21 of the 48 genera and less complex than 20 genera, leaving only 7 of the 48 genera to be rated as less distally complex as the human penis.

Gallup et al. (2003)[6] tested Baker and Bellis's (1995)[13] hypothesis that the shape of the human penis was specifically designed to scoop out human semen. Important to note is that these experiments utilized fake penises, fake vaginas and fake cum (these people literally extrapolated functionality of the human penis from dildos). They contend that the 'large' diameter of the human glans and corona have been selected for due to sperm competition. Outside of this study, there is no other evidence to support the view that the shape of the human penis has evolved to displace sperm (anyone who has studied the penises of various primates can easily see that the authors of this study suffer from a terminal case of confirmation bias). 'Acorn-like' glans (like seen in humans) is extremely common in old world monkeys regardless of whether or not they have monogamous, polygynous or multi-male/multi-female mating systems. In fact, the human penis is remarkably similar to the gorilla (whom experiences virtually no sperm competition) and differ only in size. Gallup and Burch (2004)[7] have even noted that semen displaced by rival males might be transferred to other females and thus fertilize her. If we compare Humans to more promiscuous Chimpanzee and Bonobos, it becomes very obvious that the human penis has not evolved to adapt to sperm competition. In both the bonobo and the chimpanzee a glans is lacking and the penis is filiform; the bonobo in particular has a distinctive Y-shape.[24] Measurements of penile and vaginal lengths in the chimpanzee indicate that the long and filiform penis probably co-evolved in association with the development of the large sexual skin swelling which characterizes this species.

Sexual Selection[edit | edit source]

It is certainly tenable, if not unlikely, that sexual selection has influenced the morphology of the human penis. It has been shown cross-culturally that females have a preference for larger non-erect penises.[25][26] An Australian study has found a preference for larger non-erect penises as well using 3D models.[27] However a preference for larger non-erect penis, which isn't very strong anyway, does not necessarily mean that female choice has driven human penis morphology.[28] Namely because penis size wasn't a priority for ancestral females who wanted children and hunting ability was deemed more important. Humans also wear clothing and despite our intuitions, have been wearing clothing for most of the time that we've existed. Considering that we have evidence that clothing has possibly been around since 700 000 years ago and the Homo Sapiens is only about 200 000 years old, it is unlikely that females had visual access to a males penis and thus select based on penis size. Height, for example, was a lot easier to identify and could more easily be selected for.

The are also some criticisms to be made about the studies that show a female preference for larger non-erect penis. Namely, how does the subconscious female mind differentiate between a partially erect and growing penis and a particularly large flaccid penis; partially erect penises do not stand upright like erect penises and thus are more difficult to distinguish from non-erect penises. The higher rating of larger non-erect penises might be because a larger penis in general would be a sign of arousal. The study conducted in Australia seems to indicate this, though the authors do not seem to notice.[27] It was shown that larger non-erect penises were rated as more attractive up until 8cm (3") after which any increase in size would result in diminishing returns (note that the average non-erect penis is around 8cm). A non-erect/partially erect penis larger than 8cm (3") is probably an indicator of arousal. This would nicely explain why we find diminishing returns on non-erect penises larger than 8cm (3") as the larger a penis is, the more likely/obvious it is that its owner is aroused; after a certain point it becomes extremely likely that the greater size of the penis is an indicator of arousal and thus result in a plateau of attractiveness ratings.

To illustrate, if I told you that a penis was 3 inches long without giving you any information as to whether or not said penis is erect or flaccid and asked you to estimate the likelihood that that 3 inch penis is erect, you would most likely answer that the penis is flaccid. If I asked you to do the same with a 5 inch penis you would most likely answer that the penis is erect; the higher the number goes the more likely it is that we would guess that a penis is in a state of erection. Ergo, the chance that a penis is in a flaccid state diminishes the larger the penis becomes and would thus explain why we see diminished returns on flaccid penises larger the 8cm (3").

Female Orgasm[edit | edit source]

See female orgasm

References[edit | edit source]

  1. 1.0 1.1 1.2
  3. 3.0 3.1
  6. 6.0 6.1
  7. 7.0 7.1
  13. 13.0 13.1
  14. 14.0 14.1
  17. 17.0 17.1
  18. 18.0 18.1
  27. 27.0 27.1