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the origins of the strange sight of the human penis.
    Now, the irony doesn’t escape me. But in spite of the fact that this particular evolutionary psychologist (yours truly) is gay, for the purposes of research we must consider the evolution of the human penis in relation to the human vagina. Magnetic imaging studies of heterosexual couples having sex reveal that, during coitus, the typical penis completely expands and occupies the vaginal tract, and with full penetration can even reach the woman’s cervix and lift her uterus. This combined with the fact that human ejaculate is expelled with great force and considerable distance (up to two feet if not contained), suggests that men are designed to release sperm into the uppermost portion of the vagina possible. Thus, in a theoretical paper published in the journal Evolutionary Psychology in 2004, Gallup and coauthor, Rebecca Burch, conjecture that, “A longer penis would not only have been an advantage for leaving 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.”
    This “semen displacement theory” is the most intriguing part
of Gallup’s story. We may prefer to regard our species as being blissfully monogamous, but the truth is that, historically, at least some degree of fooling around has been our modus operandi for at least as long we’ve been on two legs. Since sperm cells can survive in a woman’s cervical mucus for up to several days, this means that if she has more than one male sexual partner over this period of time, say within forty-eight hours, then the sperm of these two men are competing for reproductive access to her ovum. According to Gallup and Burch, “examples include, group sex, gang rape, promiscuity, prostitution, and resident male insistence on sex in response to suspected infidelity.” The authors also cite the well-documented cases of human heteroparity, where “fraternal twins” are in fact sired by two different fathers who had sex with the mother within close succession to each other, as evidence of such sexual inclinations.
    So how did natural selection equip men to solve the adaptive problem of other men impregnating their sexual partners? The answer, according to Gallup, is their penises were sculpted in such a way that the organ would effectively displace the semen of competitors from their partner’s vagina, a well-synchronized effect facilitated by the “upsuck” of thrusting during intercourse. Specifically, the coronal ridge offers a special removal service by expunging foreign sperm. According to this analysis, the effect of thrusting would be to draw other men’s sperm away from the cervix and back around the glans, thus “scooping out” the semen deposited by a sexual rival.
    You might think that’s fine and dandy, but one couldn’t possibly prove such a thing. But you’d be underestimating Gallup, who in addition to being a brilliant evolutionary theorist, happens also to be a very talented experimental researcher (among other things, he’s also well-known for developing the famous mirror
self-recognition test for use with chimpanzees back in the early 1970s). In a series of studies published in a 2003 issue of the journal Evolution & Human Behavior, Gallup and a team of his students put the “semen displacement hypothesis” to the test using artificial genitalia of different shapes and sizes. They even concocted several batches of realistic seminal fluid. Findings from the study may not have “proved” the semen displacement hypothesis, but it certainly confirmed its principal points and made a believer out of most readers.
    Here’s how the basic study design worked. (And perhaps I ought to preempt the usual refrain by pointing out firstly that, yes, Gallup and his coauthors did receive full ethical approval from their university to conduct this