Thursday, October 29, 2015

Human Evolution and Women’s Sexuality -- Part IV: Why Do Women Bleed?

By Kristy McCaffrey

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Human males are saddled with a sexual desire that rarely wanes. While other male species will see a spike in testosterone levels in conjunction with the mating cycles of females, human males maintain a steady concentration year-round.

Because ancient human females had such high iron needs due to the many sources of blood loss in her lifetime (menses, childbirth, and lactation), a peculiar adaptation occurred in the male—constant, high levels of testosterone. One theory proposes that this pushed the male to take ever greater risks—to hunt that giant animal that could trample and kill him—all to return with iron-rich meat for the female and her offspring, his insurance plan for sex. But the side effect of this has been a disparity in the sexual needs of men and women.

Women require significantly more iron than men. Men have incredibly high testosterone levels which create an intolerable sexual tension that demands release. These two situations are intimately linked.

In many early cultures, hunting was equated with sex. The better hunter a man was, the more opportunities he had with willing females. To state it crudely, a good hunt exchanged meat for sex. Females encouraged this further by favoring males who were the best hunters, bringing back meat to replenish her lost iron stores.

But, ultimately, it was the adaptation of time that gave males the advantage they needed to become successful hunters. And it was the females who imparted this wisdom. Humans learned to anticipate the arrival of certain animals—migrations—and to plan accordingly. They gained foresight.

Because the female had sent the male out to hunt due to her need for iron and his never-ending desire for sex, she needed to arm him with an advantage. Naturally, over time, men would take the credit for this, but women know better. Due to her menstrual cycles, her sense of time changed. She could forecast into the future, and she shared this knowledge with the men. Learning to tell time is an exceptional evolutionary development, one that humans share with no other species. (Some creatures do exhibit similar behaviors, but these can be credited to instinct.)

Humans can plan, we can choose a course of action. We understand that one day, far in the future, the sun will cease to be. We know that we exist in a three-dimensional paradigm. Even the most intelligent of other species—dolphins, elephants, and higher primates—are unable to grasp these issues.

And this is perhaps why a woman’s menstrual cycle is linked to the motions of the Moon. Or, said in another way, because her cycles mirrored a lunar one, an inherent sense of time was born within the female mind.

In many cultures, menstruation is referred to as ‘the moon’. In rural India, the moon is believed to be the ‘cause of all time’, just as it is the cause of menstruation. Long ago, menses, the moon, and the duration of a month came together and forever changed a female human’s ability to navigate a different time-space.

In his book Sex, Time and Power: How Women’s Sexuality Shaped Human Evolution, author Leonard Shlain writes, “Foresight has proved to be the sexiest idea that Mother Nature came up with since Her clever invention of the penis two hundred million years earlier. Whereas the penis significantly advanced the fortunes of every reptile and mammal species that acquired one, foresight dramatically increased the fortunes of only humans, at the expense of all other species.”

Why do women bleed so much? Shlain posits that it is so humans could anticipate the future, something no other animal had ever before accomplished.

So, back to the initial question—why do women lose so much blood each month? The answer is because it was necessary for human evolution. And what was this evolution? Linking a woman’s bleeding, an event too big to ignore, with the cycles of the moon taught humans how to tell time. The ability to practice foresight was and is the most important asset we possess as a species.

The price for all of this was the depletion of iron in human females. To combat this, they reshaped their sexuality, incorporating a system of free will when it came to choosing sexual partners, and males were forced to comply with this turn of events. Negative outcomes have been patriarchy, misogyny, and the use of rape, but positives have been males who exhibit more kindness, compassion, and love to their mates—and offspring—than any other species on earth.

To learn more about this fascinating subject, I encourage you to read Sex, Time and Power: How Women’s Sexuality Shaped Human Evolution by Leonard Shlain (Penguin Books, 2003).


  1. Interesting, Kristy. I have always thought having a period the worst thing ever. It goes on and on and one long after any normal person wants more kids. Ick. I cracked up at "the penis" as a clever invention. Only a man would describe it as such! Hugs...

    1. Hi Tanya--yes, nature has some weird tricks up her sleeve!! Thanks for stopping by.