I was originally going to ask about the evolutionary advantage for menstrual synchrony, but I was operating under two assumptions: a) it does actually happen and b) the effect is caused by pheremones. 

Based on some poking around on the internet, it is appearing that this idea (published by Martha McClintock in the 70's), is under question--people are criticizing her studies and some say that this effect does not actually occur.

So my new question is:  Is there any sort of scientific consensus?  If not, what do you all think?  Thoughts?  Speculation?

If it is plausible, why might it have evolved?  What advantage would it confer?

Thanks guys, I love this website!  :)

Well I never, you are absoultely right! We were taught this as absolute fact when I was a med student in the late 70s and its interesting to look at the primary data now and see how flawed it is! As a neurobiologist who principally works on rodents where it is for sure a proven fact - it always seemed to me entirely reasonable that by extension it would happen in female humans. A perfect example of why one should not assume anything and test every hypothesis rigorously!

The URL below, nicely summarises the issues and speculates on possible causes (assuming it does happen).
http://www.menstruation.com.au/periodpa … hrony.html

very many thanks for highlighting this issue Sara - I learned something new today!

Can't say I know anything about this at all ... but being intrigued I just had a look at the website David highlighted and what I would say is that IF menstrual synchrony does happen then some of the possible causes mentioned look a bit shaky to me.

There is a lot of "for the good of the tribe" stuff in there, which (from an evolutionary biologist's perspective) doesn't tie in too well with current thinking in social evolution.

I would say that it might well be useful in many species. If rats menstruate together and birth together, this may act as a predator defence. With large numbers of young around many may well be able to avoid predation ( a tactic often seen with birds that nest togehter or marine turtles).

With humans the opposite is true. In an evolutionary sense, women have gone to quite a lot of trouble to hide the times that they are fertile from their partners (compared to say other apes and monkeys) in order to keep them from leaving (if you don't know that your parter is fertile or pregnanat, you need to keep supporting and defending her and keep other males away). Synchronous menstruation could potentially undercut this - in a group if all females were simultaneously fertile the males would soon learn and this might end the 'disguise'.

Just some thoughts, nothing but speculation there, but it makes sense to me!

It still makes sense for human females to be fertile together if it means they fall pregnant at similar times. Humans are social animals and child-rearing is rarely the job of just the mother. Human wet-nurses exist in most human societies and are a great way to improve offspring survival, particularly when human females frequently don't survive the birth process (when modern medicines are not available). That means several women (usually related if they are in a small society) lactating at a time will improve the chances of survival for their sibling's children.

Yes in very small groups (i.e. within a family), but in a tribal situation it would not. If a male could ifdentify that the whole female population was fertile based on a single female (as would be the case with synchronicity) then his individual best strategy would be to try and father as many children as possible then leave for the next tribe while not contributing to the raising of the children. But if this remained hidden, he does not know when the female was fertile and whether or not he is necessarily the father, thus he is better off staying to help!

In principle yes, but I think you can't ignore the bigger picture about social groups. For a start there are other males in the system, so there will be competition for females. Males in a human groups form strong bonds since they tend to rely heavily upon each other in a social context for hunting and defense of the group, so conflict is bad. Roaming males are generally seen as a threat to groups (hence the general trend for females to move into the male's tribe in inter-tribal marriages). Also consider that humans have a complex social structure. Status determines mating opportunity, and new members of a group tend to enter at a low status. Plus, females exercise choice and will generally require extended courtship before mating. A roaming male is at a huge disadvantage, so multiple mating and moving on has strong limitations as an effective strategy for humans. Many of these social mechanisms may have evolved as a way of preventing exactly this kind of behaviour, since the best strategy for an individual males is not the best strategy for a species with altricial, slowly maturing offspring. Generally, male reproductive strategy is strongly driven by female mate choice, which in turn is driven by reproductive constraints determined by the environment.

Last edited by Paolo Viscardi (28th Sep 2007 06:57:22)

I think that the idea that women would fall pregnant at the same time is a bit of a red herring here.  Assuming we have a primitive group where nutritional needs are being met to the point where all women actually do ovulate/menstruate every month, we probably also have a group where women become pregnant again soon after lactation ends.  In this case, pregnancy can occur every 24 months or so (maybe 18-36 months), while menstruation occurs every month.  These numbers are so different, that it really doesn't matter if menstruation occurs simultaneously, there still isn't going to be a "birthing season" based on menstruation timing.  Add this to the wiggle room in gestation (two weeks on either side of 38 weeks) and synchrony in the 4-week menstrual cycle looks like it isn't going to do a lot in terms of birth timing.

Fascinating stuff!

I was under the impression that under laboratory conditions rats are generally much less sensitive than mice to synchronisation of cycles with cage-mates so I suppose synchronicity will be quite species (and strain?)-specific as well.  Some reproductive behaviour e.g., the Bruce effect, where pheromones from an unfamiliar male can block implantation of embryos into the uterine walls of a recently bred female, is apparently more pronounced in mice than rats, and is dependent in part on the action of hormones such as vasopressin and oxytocin in the brain. Other behaviours like the Whitten effect (cycle synchronization of females exposed to male pheromones) seem to be more pronounced in rats than mice. I guess cycle synchronicity also depends on whether you are a monogamous or polygomous species as well!

Last edited by Steve Lolait (11th Jun 2010 12:47:38)