Menses and the Lunar Cycle

Cecil’s latest article, http://www.straightdope.com/columns/990924.html , says:

What an experiment like this might fail to take into account is LIGHT CUES. Remember that experiment somebody-or-other did several years ago, where he shut himself up in a cave to see what his daily rhythms would do when deprived of light cues from the sun? He discovered a daily circadian rhythm existed independently of daylight/nighttime cues with a cycle period of 25 hours. Only when exposed to a dawn-and-dusk rhythm from the outside world does the human body sync up to a 24-hour daily cycle.

SO … it’s conceivable that the same thing could be happening in the human female menstrual cycle. Back in the Good Old Days before we humans invented campfires, the only significant light source at night was the moon. The human menstrual cycle may have evolved to take advantage of that, what with it being easier and safer to mate in dim light than in no light. Nowadays, though, with our nights bathed in streetlamps and house lights, there are no light-cues for the female reproductive system to synchronize with, so it would fall back to a seemingly random cycle whose period (pardon the expression) only appears coincidentally similar with the lunar light cycle.

The only way to tell for sure, of course, would be to get a bunch of women to agree to be exposed to NO light at night other than moonlight, and see what happens to their menstrual cycles. Which sounds like an expensive experiment.


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Why do humans persist in believing they are ‘special’ among the animals? Evidence of menses cycles among other mammals shows no trend towards developing a monthly cycle. Why would humans be different?

As for the experiment mentioned, unless significant numbers are involved, the data would be inconclusive. And who would want something like 1000 women to be alone without males around for a lengthy period of time? They might develop parthenogenisis, for goodness sake, and THEN where would we males be?? :wink:

Oh noooo, not again! Why are (some) guys so fascinated with this topic?

There is one way that humans are different from most other animals. We don’t just mate when the female of the species is fertile. We mate at any time, for pair-bonding, or just because it’s fun. We females don’t even give you males any clue about when we’re fertile - which is only fair, since most of us don’t automatically know it either. So synchronizing menses to one part of the month wouldn’t do anything towards limiting sex to any particular part of the month.

Besides, why ascribe quasi-Victorian morals to our ancestors? Maybe they didn’t care if the lights were out and the shades were drawn. Maybe the nearest boulder and the dark of an ordinary night were privacy enough. Heck, maybe they didn’t care if others were watching. Life was different in those days, and they didn’t exactly leave many clues about their sex lives behind for modern archeologists to discover.

Guys? Most guys I know have learned to avoid the topic of menstration if there are women around, because they’ll tend to get teased for their ignorance and general squeamishness about the subject.

And anyone who’s tried to talk rationally with someone who believes her body is in tune with the cycles of the moon tends to get gun shy on that subject too.

So, I guess the guys you know must have real thick skins.

I’ve known many women who put a lot of stock in the notion that their bodies were in phase with the lunar cycle. I’ve also heard it used as evidence that women were more in tune with nature. Obviously, the fact that estrus cycles vary greatly among mammals make that idea absurd.

Unless, as DSYoungEsq points out, you believe that humans are special in that regard. Ironically, the idea that humans are special among animals is one that will probably be abhorent to the sort of people who believe this moon/menstral cycle stuff to begin with.

flodnak wrote:

Most other animals, but certainly not all others. Bonobos (close relatives of Chimpanzees) are notorious for their promiscuity. They boink so often they make humans look positively Vulcan by comparison.

However, bonobos don’t pair-bond with their mates. Most species don’t pair-bond, in fact. Wolves do. Many fish species do. Many bird species do, although recent research has shown that chickadees only appear monogamous. And of course, we homo sapiens have been known to “cheat” on our pair-bonded mates and even get divorced from time to time.


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In all the discussion of menstral cycles, I’ve never heard anyone suggest the following theory (my own): in pre-artifical light ages, it’s easier to see what you’re doing and find your mate after the sun sets when there’s a full moon out.

And why are other mammals’ cycles not 28-29 days? Because humans are more visually dependent than most mammals. The original timing of menstrual cycles in our remote ancestors may have had entirely different causes, but for humans it was slightly advantageous to have the maximal fertility time match the time of highest visibility.

“Oh noooo, not again! Why are (some) guys so fascinated with this topic?”

Several answers come to mind:

  1. You notice it more when a man is interested. I’ve spoken with more women about the subject than men.

  2. Women are generally less interested than men because it’s
    a) Routine for them
    b) An annoyance

  3. Men are generally fascinated about anything to do with women, especially the differences between women and men.

  4. It’s something unknown, which is ipso facto interesting to many men. It’s far from certain that women’s menstrual cycles are in any way connected with lunar cycles.

“Why do humans persist in believing they are ‘special’ among the animals?”

Try “different” rather than “special”.

In many ways, humans are different to other animals. In many ways they are the same or similar. Every species is, to use your term, special in one way or another.

Or maybe we just went through a period in our evolution where we were really ugly, and it was easier to mate in the dark :).

But really, saying that we needed the light doesn’t make a lot of sense, since there’s no reason not to mate during the day. Unless the reason does center on darkness rather than light…

Another possible rationale: It’s already known that women living together tend to synchronize, and a variety of evolutionary rationales have been proposed for this phenomenon. Regardless, though, it’s pretty clear that it’s advantageous (for whatever reason) for females to synchronize. What if the light levels from the Moon aren’t directly important at all, but serve as an outside stabilizing factor for synchronization? That is to say, even if females aren’t living together, if they all become synched to the Moon, then they’ll all be synched to each other, as well.

An odd aside, anectdotal, but perhaps of interest in dealing with light cues: my menstrual cycles are very regular. Aside from the moon cycle , mine tend to be when the moon is waning, if not dark.; my cramps always start with the first light of day. Maybe this is just the point of the body waking up, and starting all functions, but it’s been my body pattern for decades. And it varies with the seasons, according to sunrise. First inkling of light=bad cramps. I’ve lived in rural areas for some time, so am cued to natural light. That might have something to do with ovulation being around full moon too. Dunno, but that’s my pattern.

That’s not as well-established as was once thought:

Damn,Chronos beat me to it. I was gonna ask why you people keep assuming our ancestors mated at night. Sure, that’s a common pattern now, but our lives are structured around a totally different lifestyle than our pre-human ancestors.