It is a curious phenomenon that women who live or work in close proximity often menstruate at the same time. Somehow, over time, their cycles synchronize. What's the explanation?

It's pheromonal.

Pheromones.

Women release certain hormones into the air. Other women breathe them in, and their bodies synchronise cycles.

Imagen a doomsday devise that would unleash those pheromones globally.

Which one gets to decide who they "sync" to though? Whose example do they all follow? Does it just line up with lunar cycles etc?

Imagen a doomsday devise that would unleash those pheromones globally.

Damn.

Rush Limbaugh, in one of his books, imagined a brigade of female army soldiers who would be unleashed on the enemy once a month.

Also, what is the genetical advantage of it? I would say different times are better if only a few males are aviable, because they can keep impregnating the ones having their fertile periods. If all/most women get their fertile times at the same time, the chances getting pregnant will decrease...

Probably a genetic "compitition" thing.

It does happen and it DOES SUCK. :(

Which one gets to decide who they "sync" to though? Whose example do they all follow? Does it just line up with lunar cycles etc?

Apparently, in the pheromone wars, some women just naturally dominate others.

Also, what is the genetical advantage of it? I would say different times are better if only a few males are aviable, because they can keep impregnating the ones having their fertile periods. If all/most women get their fertile times at the same time, the chances getting pregnant will decrease...

Women want to be able to choose who they have children to. A good way to do that is for women living together to all sychronise their fertility cycles. That way, they all become fertile at the same time, and the women can each choose from the available men who they'd prefer to breed with.

The opposite situation, in which each woman becomes fertile at a different time, gives all the males the opportunity to concentrate their energies on a single female at a time, which potentially takes away her choice. It is also more likely to lead to conflict among the males, though genetically speaking that is probably a minor consideration.

Note that this kind of thing evolved when humans lived in relatively small groups, numbering probably less than 30 individuals on average. In today's world, the advantages are less obvious, but evolution is a slow-moving thing.

It's pheromonal.

And before they discovered pheromones, those women noting these patterns were not doubt dismissed as basing their ideas on anecdotal evidence and or engaging in wishful thinking. Thousands of years after the phenomenon is noted by women, scientists come up with 'proof' and a 'cause'. I wonder what other phenomena scientists now consider simply the anecdotes of irrational people that we (as a species) will find to be 'proven' one hundred or a thousand years from now.

The opposite situation, in which each woman becomes fertile at a different time, gives all the males the opportunity to concentrate their energies on a single female at a time, which potentially takes away her choice. It is also more likely to lead to conflict among the males, though genetically speaking that is probably a minor consideration.Dogs are just the opposite. They tend to space out their estrus cycles so that no two females are in heat at the same time. (When it's twice a year instead of 28 days they can do that.) The female deliberately tries to mate with all the males. That way when her puppies are born, every male will figure that at least one of them might be his, and he will protect them. Social sex is also a mechanism for strengthening the bonds among the pack members.

The opposite situation, in which each woman becomes fertile at a different time, gives all the males the opportunity to concentrate their energies on a single female at a time, which potentially takes away her choice.

Sorry if I'm slow, but how does this give women more choice? It seems that this gives them MORE choice because they can pick the best male from those wooing her. Also, having all of the women fertile at the same time seems to mean that each male should have a harem, so to speak.

Maybe, if one guy has a pack of females, if the females all get pregnant at the same time, raising childeren (who would all be the same age) is easier?

I doubt it has much to do with a Darwinian "strategy". At a guess, it is more along the lines of a system of weakly coupled low q resonators. More physics than biology.

Hormonal gain. ;)

My apologies to any ladies present, but I couldn't resist. Bad kevin!

There you go! All kidding aside, I bet you could model it along those lines.

Exactly. It probably wouldn't be to hard to do either. Each woman represented as an astable with an average period of 28 days. Add some randomness (not all time constants 28 days). Throw in a retrigger minimum period and feedback network and I bet you would have pretty good model.

Reading over this, I should perhaps mention for non electronics female types that the term "astable" was not a dig or insult, it's an electronic circuit, usually in ic form.

Just a thought. Why is it that female cats and dogs do not menstrate. No other female specie menstrates, that I know of... for that matter.

Just a thought. Why is it that female cats and dogs do not menstruate?

They do!

It is usually refered to as a female animal being in heat. Most animals only cycle once or twice a year though. It's species dependant. Humans are kind of unusual in cycling once a month.

Ever try to live in the the same house with a house cat in heat? It'll drive you up the walls. Constant howling. She got demoted to outdoor cat real quick. I think that's where the word "caterwailling" comes from.

When all the females in a group breed, and have children at the same time, then ALL the children that are born have a higher survival rate. Power in numbers!