Thursday, March 08, 2012

Why didn't menstrual synchronicity evolve out of existence?

I have experienced menstrual synchronicity myself (with my sister, with my cohort in university, with co-workers), although I have no way of telling if it was coincidence or not. However, it occurs to me that evolution should kill menstrual synchronicity.

If every female in the group is menstruating at the same time, that means they're also all fertile at the same time. Which means that, outside of that one fertile window, any sex that is had in the entire group is wasted (from a survival-of-the-species point of view - qualify everything I say in this blog post with "from a survival-of-the-species point of view"). And if the male-female ratio is such that not all females can get all the sex necessary to become pregnant during the fertile window, a whole menstrual cycle is wasted.

But if there's a female who isn't synchronized with the rest of the group, she can get pregnant when no one else can. If males are at a premium (especially given the trends observed in our primate ancestor of females being more inclined to mate while fertile and males being more attracted to fertile females), she has her pick of all the males during her fertile window, as opposed to having to share with the rest of the group, thus increasing her chances of becoming pregnant

Even if there is no evolutionary disadvantage to synchronicity, it seems like there is a bit of an advantage to asynchronicity.

Of course, there's also the fact that, for most of evolutionary history, human and primate females spent most of their time either pregnant or lactating, and therefore unable to become pregnant anyway, which makes the impact of menstrual synchronicity or asynchronicity seem tiny. But, on the other hand, evolution has taken places over millions and millions of years. And millions and millions of tiny impacts can add up to something significant.


laura k said...

I've experienced it, too. It's very msyterious.

(And women can and do become pregnant while lactating.)

M@ said...

I wonder if it's a matter of looking at it from the correct direction. In a general sense, the population of the species benefits from the maximum amount of fertility for the maximum amount of time -- I think that's the way you're looking at it (and I may be misinterpreting you there). But the key to genetics is that it is the individual genes that are competing to survive, not the species.

I think menstrual synchronization should be looked at as a competitive advantage for the individual organism. A female would synchronize to ensure that while they're infertile, everyone else is infertile, too -- ensuring that the partner male cannot go around fertilizing others while the partner female is infertile. I think it needs to be thought of as a means of denying other females an advantage, rather than a general advantage for the population.

One thing that would help to clarify this is whether the synchronization occurs with species that organize differently, i.e. non-monogamous species.

laura k said...

I think M@ nailed it there. Competitive advantage over other females, just like male plumage in other species.

* * * *

Totally OT, but just in case you didn't see it:
Last night at NYC's Apollo Theatre

Wow, seeing Bruce in that venue would be something.

impudent strumpet said...

I question whether the females would be evolutionarily driven to hinder the other females' attempts at reproduction. It seems to me that if evolution worked this way, we'd be hormonally driven to kill other people's babies rather than protect them. And I can tell you that, for me at least, I have an instinct to protect babies coming from the same place that produced a hormonal yearning to have a baby back when I wanted to have a baby. (Of course, that opens us up to the possibility that I might grow out of my instinct to protect babies, which would be weird.)

I guess it's a question of whether the females are driven to reproduce specifically with their mate (if they're pair-bonded), or with the best available male (in which case being asynchronous might be an advantage if you wouldn't normally have access to the best males), or just to reproduce at all with anyone. And I don't know the answers to these questions.

impudent strumpet said...

(And women can and do become pregnant while lactating.)

THAT is WEIRD!!! I wonder how the human body can possibly survive gestating and lactating at the same time, especially if you're living in a context where food isn't abundant??

laura k said...

It seems to me that if evolution worked this way, we'd be hormonally driven to kill other people's babies rather than protect them.

I don't think one follows the other. Females can have the drive to suppress other female's ovulation and still have the drive to protect the young.

The instinct to protect babies seems to be in all mammals and it works across species - all this vids of animals taking care of orphans of other species.

The beta and lower female wolves babysit the pups while the alphas are out hunting. All the wolves in the pack protect the young - but the alpha's dominance suppresses estrus in the other females.

impudent strumpet said...

I don't see how preventing one's partner from fertilizing other females prevents the other females' ovulation??

laura k said...

You mean in the wolves? The alpha female doesn't prevent the alpha male from mating with the other females. She doesn't have to! Her power forces the other femals to not go into estrus. So she's the only one in heat, which means she's the only one who will select a mate. All the males are only interested in her.

AFAIK, it's not understood how she manages this. She's the biggest and the most beautiful female, because she's eaten the most, and she got that way from being strong and fearless, dominating all the other females. And for some reason, this includes their reproductive cycles.

The beta females all help take care of the pups.

The whole thing is quite amazing (to me).

impudent strumpet said...

Oh, I thought you were talking about the same thing M@ was talking about. Suppressing others' ovulation is like a superpower! It's like something an X-men character would have or something!

laura k said...

It is a super power! Female alpha wolves are AWESOME. (Wolves in general are awesome to me.)

But this also goes to what M@ said. If (a) there's already a pair-bond between a female and male and (b) all the females are ovulating at the same time, then it follows that menstrual synchronicity will increase the likelihood of that pair mating, that male not mating with other females, and so the female's genes standing a better chance of moving on.

And if the male does mate with any of the other females, chances are better that the "other" (non-pair-bond) female will get pregnant.

I don't know if I'm explaining it right.

laura k said...

And if the male does mate with any of the other females, chances are better that the "other" (non-pair-bond) female will get pregnant.

Sorry, will ^not^ get pregnant.

impudent strumpet said...

I just don't see how, if the male mates with non-pair-bond females while his partner isn't ovulating, how that would have any impact whatsoever on the female mate's genes getting passed on. He'd still mate with his partner while she's ovulating (if he didn't, they wouldn't be mates) so her chances of getting pregnant are just the same.

The only exceptions I can see is if it's a species like penguins where both the male and the female need to be immediately involved in the care of the young. Or perhaps if the male is only capable of sexual performance once a month.

laura k said...

There would potentially be offspring from a different female.

In wolves, that would seriously jeopardize the alpha female's status. There can only be one litter at a time. The female that gives birth is the alpha. Another female getting pregnant means the alpha female is overthrown. It's really bad for the whole pack - causes a lot of tension and instability.

In humans, it would potentially move the male away from one female to another, because he'd have to support his offspring. Or it would leave one female with offspring and no male, which is hunter-gatherer days is very bad.

impudent strumpet said...

I think part of the problem is we're all tacitly thinking of different animals. I was thinking more of primates (without having any specific primate in mind), where the males aren't particularly relevant to the babies' survival.

With the wolves, do they have some way of avoiding incest? If all the wolves in a given territory have the same mother, it seems like they'd have a genetic diversity problem.

laura k said...

"where the males aren't particularly relevant to the babies' survival."

I don't know much about non-human primates, I didn't realize that was the case. I only know about wild canids. So yeah, it's probably different for different animals.

Re wolves, I've only seen things (like this) about the genetic diversity of a wolf population across packs. Within packs, there doesn't seem to be a problem (I don't know why), but if all the packs are closely related, then there would be.