What Penis Size Has to do with Monogamy

May the best sperm win

In the long arc of human anthropology, we have not always been monogamous. This may come as a surprise to you.

In earlier times in human history, women were known to seek several partners when they were ovulating, which improved their chances of healthy offspring — the best gift we can give the next generation.

While OB-GYNs joke that cervixes are good for two thing, having babies and getting cancer (No joke. My beloved OB said this to me 🙄), the cervix has a curious lesser known role: sifting semen.

In case you’re a little fuzzy on where the cervix is (sex-ed was a looong time ago, and probably didn’t teach you anything worth remembering, much less about the female reproductive system), you’ll find it at the end of the vaginal canal, at the opening of the uterus.

When our early female ancestors took multiple lovers, the cervix helped to sift the semen, allowing the best sperm through. She’s a gatekeeper. And the egg herself selects the sperm.

…sperm have only one job—to fertilize eggs—so it doesn’t make sense for them to be choosy. Eggs, on the other hand, can benefit by picking high quality or genetically compatible sperm.

“The idea that eggs are choosing sperm is really novel in human fertility,” said professor Daniel Brison, the scientific director of the Department of Reproductive Medicine at Saint Marys’ Hospital.1

This history of non-monogamy is why women are more vocal than their male counterparts during sex. While our ancestors were getting it on in the jungle, the female partner’s cries would draw other potential partners close.

Like the poet Kelis said:2

“My milkshake brings all the boys to the yard”

As members of the primate species, our closest relatives are bonobos: we share 99%3 of the same DNA sequence.

Bonobo photo by Martin Surbeck

Bonobos are sexually prolific. They engage in both heterosexual and homosexual sex dozens of times a day (yes, you read that right): when they are fed, when they get excited, when newcomers arrive. It’s a way to bond, to alleviate tension. Unlike human copulation which lasts for an average of 5.44 minutes (may the record show, this is a more reductive definition of sex, from penetration to ejaculation), bonobos’ exploits last an average of 135 seconds.

Female bonobos give birth on average every five to six years, which means that they, too, have differentiated between sex for procreation and sex for pleasure.

Back to penis size.

The mushroom shape of the head of the penis is cleverly designed as a functional tool to scoop out any ejaculate from rival males that may still be present in the vagina.

“The mushroom configuration of the human penis evolved as an adaptation to female infidelity,” says Gordon Gallup, PhD, who popularized the so-called semen displacement theory with a co-authored study, published in 2003.

“If a female has been inseminated by another male within a brief period of time, the next male in sequence has the opportunity to scoop out or displace the prior male’s semen from the female’s reproductive tract, so he can substitute his semen for those of his rivals,” says Gallup. “It’s called the semen displacement hypothesis.”

“Deeper thrusting displaces more semen than shallow thrusting,” says Gallup. “And males with longer penises can deposit their semen in the most remote parts of the female vagina, therefore making displacement by [later] males less likely.”6

They’ve even done a test with dildos, a vagina sleeve, and a concoction that mimics the consistency of sperm. “They found that dildos featuring a coronal ridge (like that of a real penis) displaced 91 percent of semen present in the vaginal canal.”7

Human testis size indicates that males evolved under conditions in which their sperm competed inside females, but perhaps not to the same extent as chimp sperm. But the larger human penis suggests that hominids needed to keep females with choice sexually satisfied. Ancestral females would have experienced a sexual freedom denied in Western cultures today and it has been suggested that our ancestors went through a period of matriarchy and enhanced female choice.

Carole Jahme, The Guardian

When we consider our history, it’s clear that human sexuality is complex and influenced by many factors outside of the physical (which certainly doesn’t hurt).

While penis size may play a role in female pleasure, the kind of sexual satisfaction we crave includes a lot more than what a ruler can measure: emotional connection, safety, and communication.

At the end of the day, we know that only about a quarter of women experience orgasm from penetration alone. So while size matters, female orgasm isn’t contingent upon it.

Intimacy is so much more than just anatomy.

Sex and Style is written by Certified Somatic Sex and Relationship Coach and Wardrobe Stylist, Sarah Ward. She has spent the last 20 years studying human sexuality and minted it in 2021, certifying in the VITA™ Methodology with Layla Martin, and as an Erotic Blueprints™ Coach with Jaiya. Sarah was personally mentored by one of 10 Certified Image Masters in the world. She has styled over 5000 photo shoots and dressed thousands of bodies. Her work has been published in Seattle Met Bride and Groom and Women’s Wear Daily.

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