The human penis as a semen displacement device

https://doi.org/10.1016/S1090-5138(03)00016-3Get rights and content

Abstract

Inanimate models were used to assess the possibility that certain features of the human penis evolved to displace semen left by other males in the female reproductive tract. Displacement of artificial semen in simulated vaginas varied as a function of glans/coronal ridge morphology, semen viscosity, and depth of thrusting. Results obtained by modifying an artificial penis suggest that the coronal ridge is an important morphological feature mediating semen displacement. Consistent with the view of the human penis as a semen displacement device, two surveys of college students showed that sexual intercourse often involved deeper and more vigorous penile thrusting following periods of separation or in response to allegations of female infidelity.

Introduction

To maximize the likelihood of paternity, human males have evolved several strategies. Daly and Wilson (1998) list mate guarding, as a means of minimizing the chances of the female becoming involved in an extra pair copulation, and using the degree to which children show paternal resemblance to assess paternity. There is a third class of putative paternal assurance tactics. In the event of extra pair copulation, strategies on the part of the resident male for preventing conception by an interloper may involve different parameters of sperm competition. Baker and Bellis (1989) have shown that the less often a man sees his partner over a 3-day period, the higher his sperm count upon resuming sexual intercourse, implicating mechanisms that may function to compete with sperm of rival males.

While the first few drops of ejaculate facilitate sperm transport, the last few have spermicidal properties (Baker & Bellis, 1995). Therefore, in the event of a double mating, the second male's sperm would be met with a spermicidal layer left by the first. Men with prostate or seminal vesicle dysfunction sometimes have a reversed seminal fluid sequence and are often rendered infertile (Kvist, 1991). The first few drops of ejaculate also function to protect sperm from spermicide left by previous males, and therefore improve the second male's chances of achieving conception (Lindholmer, 1973).

Species differences in genital morphology can arise by sexual selection Dixson, 1987, Eberhard, 1996, Verrell, 1992. Thus, the morphology of the human penis itself may have been influenced by sperm competition. Relative to other primates, the human penis is larger in both width and length, roughly twice as long and wide as that of our closest living relative, the chimpanzee (Short, 1980). The shape of the human penis also differs from many other primates because the glans is more exaggerated and uniquely configured (Izor, Walchuk, & Wilkins, 1981). The diameter of the posterior glans is larger than the penis shaft itself, and the coronal ridge, which rises at the interface between the glans and the shaft, is positioned perpendicular to the shaft. The human penis may displace seminal fluid from other males in the vagina by forcing it back around the glans. The effect of thrusting, according to this analysis, would be to draw foreign semen back away from the cervix. Because of its elasticity, the vagina expands around the penis during intromission creating suction that may further enhance this effect. If a female copulated with more than one male within a short period of time, this would allow subsequent males to “scoop out” semen left by others before ejaculating (Baker & Bellis, 1995).

Although there are no empirical data that bear on the semen displacement hypothesis, some behavioral implications of sperm competition in humans have been examined. Shackelford et al. (2002) found that after separation, males in committed relationships rate their partners as more attractive, show greater interest in copulating with them, and think that their partners are more sexually interested in them.

To test the displacement hypothesis, we measured the magnitude of semen displacement affected by various artificial genitals, different simulated semen recipes, different semen viscosities, and different depths of thrusting. Two surveys were also conducted to corroborate and extend the results obtained using laboratory analogs.

Section snippets

Experiment 1

In the first experiment, sexual encounters were simulated using artificial genitals and the magnitude of artificial semen displacement was measured as a function of penis size, shape, and depth of thrusting.

Experiment 2

Using a different artificial vagina and a different semen recipe, a second experiment was conducted to replicate and extend these effects. Displacement was measured across two levels of semen viscosity, and the coronal ridge was removed from one phallus to assess the effect on semen displacement.

Survey 1

The results obtained with prosthetic genitals suggest that the magnitude of in vivo semen displacement would also be expected to vary as a function of the depth of thrusting. An anonymous survey was used to determine if allegations of female infidelity affect parameters of intercourse in ways that are consistent with the results obtained using inanimate analogs.

Survey 2

Another survey was used to determine if periods of separation between couples change sexual behavior in ways consistent with the displacement hypothesis.

Discussion

Simulating a sexual encounter in vitro, we found that phalluses with a glans/coronal ridge configuration that approximated a human penis resulted in appreciable displacement of simulated semen. Depth of thrusting was also an important parameter, with significant displacement occurring only when the penis was inserted 75% or more of the way into the vagina, forcing the semen under the frenulum and causing it to flow back around the shaft and collect behind the coronal ridge (see Fig. 2).

A

Acknowledgements

We thank Steven Platek, Jack Maser, Aaron Goetz, Nicholas Pound, and Todd Shackelford for helpful comments on an earlier version of this paper, and Brian Quintus, Todd Shackelford, and Aaron Goetz for the flour and water semen recipe used in Experiment 2.

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    Semen-displacing copulatory behaviors (e.g., deeper, more vigorous copulatory thrusts) may function as sperm competition tactics in humans (Gallup et al., 2003; Pham et al., 2013). Using artificial genitals, Gallup et al. (2003) found that the morphology of the human penis appears to be capable of displacing rival semen from the female reproductive tract. Additionally, the post-ejaculatory refractory period may function to prevent displacement of own recently-ejaculated semen (Gallup & Burch, 2004; Gallup, Burch, and Mitchell, 2006).

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1

Now at Colby College, Waterville, ME, USA.

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