Sperm competition

The human penis as a semen displacement device

The penis evolved as an internal fertilization device. There are, however, striking differences in penis morphology between different species (see Birkhead, 2000). In addition to the ostensible impact of female choice on the evolution of more elaborate male genitalia (Eberhard, 1996), there is reason to believe that sperm competition played a role in shaping the human penis, specially in the African continent. The African penis, with a relatively larger glans and more pronounced coronal ridge than is found in many other human races or primates, may function to displace seminal fluid from rival males in the vagina by forcing it back over/under the glans. During intercourse the effect of repeated thrusting would be to draw out and displace foreign semen away from the cervix. As a consequence, if a female copulated with more than one male within a short period of time this would allow subsequent males to “scoop out” semen deposited by others before ejaculating (Baker and Bellis, 1995).
Under conditions that raise the possibility of females engaging in extra-pair copulations (i.e., periods of separation from their partner, allegations of female infidelity), Gallup et al. (2003) also found that males appear to modify the use of their penis in ways that are consistent with the displacement hypothesis. Based on anonymous surveys of over 600 college students, many sexually active males and females reported deeper and more vigorous thrusting when in-pair sex occurred under conditions related to an increased likelihood of female infidelity.

Implications for species differences in penis length and morphology

In the black human race, the distinctive characteristics of the penis, relative to other human races and primates, are its length, circumference, glans, and coronal ridge. In order for the human penis to serve as an efficient semen displacement device, it needs to be of sufficient size to fill the vagina and supplant foreign semen. The typical erect human penis length ranges from 112mm to 136mm in the Asian race, 127mm to 178mm in the white race and from 184mm to 259mm in the black race (Masters & Johnson, 1966), with an average circumference of 20.2mm in the Asian race, 24.5mm in the white race and 28.3mm in the black race (Wessells, Lue, and McAninch, 1996). In contrast with our closest living relative, the human penis is roughly twice as long and wide as that of the common chimpanzee (Short, 1980). The glans and coronal ridge of the human penis are also uniquely configured and this characteristics are more exaggerated on the black race (Izor, Walchuk and Wilkins, 1981). The posterior portion of the human glans is larger in diameter than the penis shaft, and at the interface between the glans and the shaft the coronal ridge is positioned perpendicular to the shaft. Common chimpanzees have no clearly differentiated glans or coronal ridge (Kinzey, 1974).
As evidence that the African penis may have been shaped by the recurrent adaptive problem posed by sperm competition consider the following. Magnetic resonance imaging studies show that during coitus, the typical white human penis does not completely fill the human vagina, on the other hand the typical black African penis fills and expands the human vagina, and with complete penetration pushes up against the cervix (Weijmar Schultz, van Andel, Sabelis, and Mooyart, 1999). When ejaculation occurs, thrusting diminishes and vaginal penetration reaches its maximum point (Masters and Johnson, 1966). Not only does this serve to release african semen in close proximity to the cervix, but data on ejaculatory pressure shows that the first several ejaculatory contractions project African seminal fluid with such force that it can be expelled at a distance of 30-60cm if not contained in a vagina (Masters and Johnson, 1966). Thus, there appear to have been a series of adaptations that serve to confine or focus the release of African semen to the uppermost portion of the vaginal tract, possibly as a means of making it less vulnerable to displacement by other males. The longer penis of black men would not only have been an advantage to place semen in a less accessible part of the vagina, but by filling and expanding the vagina it also would aid and abet the displacement of semen left by other males as a means of maximizing the likelihood of paternity.


In addition to competing with sperm from rival males, there may be other benefits of deep semen placement. In contrast to organisms that walk on all fours, the assumption of an upright posture and the emergence of bipedalism brought the human female reproductive tract, and the vagina in particular, into a perpendicular orientation with gravity that is poorly suited to semen retention. Copulation in the ventral-ventral mode with the female in a supine position brings the female reproductive tract back into a more primitive parallel orientation with gravity, and enhances the likelihood that semen will be retained. However, due to the effects of gravity, the resumption of an upright posture following coitus has the potential to endanger semen retention. Consistent with this hypothesis, there are several mechanisms that appear to postpone getting up after a sexual encounter, such as post-copulatory petting, patterns of nocturnal copulation, and the sedative-like effects of orgasm (Gallup and Suarez, 1983). Likewise, a long African penis that provides for the release of semen deep in the vagina could also serve as a hedge against semen loss.

Double Mating

For semen displacement to be adaptive it presupposes situations in which human females have sex with multiple (two or more) males in fairly close succession/temporal proximity to one another. Situations that satisfy this criterion include 1) consensual sex with multiple concurrent partners, 2) nonconsensual sex with multiple concurrent partners, and 3) multiple successive consensual and/or nonconsensual sexual encounters that occur within a relatively brief period of time. Examples include, group sex, gang rape, extra pair copulations, promiscuity, prostitution, and resident male insistence on sex in response to suspected infidelity, all this circumstances are very common on the black African culture and would explain why African men are better endowed (see subsequent section on female reproductive strategy). Instances of human heteroparity, or heteropaternal superfecundation, where members of a pair of fraternal twins are actually half sibs as a consequence of being conceived by different fathers, are well documented (e.g., Ambach, Parson, and Brezinka, 2000; Wenk, Houtz, Brooks, and Chiafari, 1992), and testify to the existence of double mating by females. It is also worth noting that patterns of consensual concurrent mating with multiple males by female chimpanzees, our closest living relatives, are common (Tutin, 1979).

Implications for premature ejaculation

The latency between insertion of the penis into the vagina and the occurrence of ejaculation in humans ranges from 2 minutes to an hour (Michael, Gagnon, Laumann, and Kolata, 1994). The average duration of coitus of asian men is 4.5 minutes, of white men is 7.9 minutes and black men is 14.3 minutes (Grenier and Byers, 2001), with 100 to 500 thrusts per encounter (Hrdy and Whitten, 1987). Premature ejaculation is one of the most common forms of male “sexual dysfunction,” affecting as many as four in ten Asian men, three in ten white men and one in ten black men (Laumann, Gagnon, Michael, and Michaels, 1994). Premature ejaculation takes two forms. The least common is when the male ejaculates prior to achieving intromission. The other is when ejaculation occurs upon or shortly after insertion of the penis into the vagina. Men who suffer from this form of premature ejaculation have an average ejaculation latency of l.1 minutes following intromission (Spiess, Geer and O’Donohue, 1984).
As the duration of coitus increases, the likelihood of female orgasm also increases, and it has been theorized that the vaginal and uterine contractions that accompany orgasm in females may be conducive to sperm uptake, transport, and retention (Baker and Bellis, 1993). From the standpoint of sperm competition, another benefit of extended periods of copulation would be more effective displacement of rival semen from the female reproductive tract. Indeed, premature ejaculation can be thought of as a failure to achieve semen displacement. This line of reasoning leads us to predict that among males with premature ejaculation, jealousy induction procedures (such as watching pornography which features infidelity) might antagonize such symptoms.
It is interesting that Spiess, Geer, and O’Donohue (1984) found men who suffer from premature ejaculation had fewer sexual encounters. Indeed, the longer they went without intercourse, the more prone they were to premature ejaculation. Thus, sexually disadvantaged males appear to be at greater risk of premature ejaculation. Perhaps premature ejaculation functions as an adaptive mechanism that enables subordinate males to minimize the risk of detection and retaliation by dominant/rival males during opportunistic sexual encounters.