"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)."
“The length of the foreskin is one of the most variable features of the human penis. When most uncircumcised males achieve an erection it pulls the foreskin back over the glans and back down the shaft of the penis, enabling the coronal ridge to do its business and scoop rival males’ semen away from the woman’s cervix.”
