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. The human penis, with a relatively larger glans and more pronounced coronal ridge than is found in many other 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).
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To test this hypothesis, Gallup, Burch, Zappieri, Parvez, Stockwell, and Davis (2003) simulated sexual encounters using artificial models and measured the magnitude of artificial semen displacement as a function of phallus configuration, depth of thrusting, and semen viscosity.
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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.
Í huga mér sé ég mann, sem hefur nýverið tekið saman við gamla kærustu, munda lók sinn einsog skóflu og ráðast til atlögu. Djöfull eru þessir limir til margra hluta nytsamlegir.