periods at the same time.
 

Why do women that live together or spend an exorbinate amount of time together tend to menstrate at or around the same time. I suspect its a supply and demand thing from our hunter gather days. Women always are in control of the supply of sex there for they are incontrol of sex. Say that a tribe of hunter gathers are living together and the alpha male is horny but his normal mate is menstrating he would just go to another woman and get what he has to get out of his system but if all the females are mestrating at the same time in the tribe it forces the man to monogomus (or at least more so). This is just some randomness that came into my head today.

Yes, it's an evolutionary thing. This is one of many ways females compete for reproductive conditions.

For the record, human females somewhere along the way also gained the ability to hide their cycle from males (compared to females of other species who display ovulation quite prominently), giving them another advantage in terms of seeking male-based support regardless of where they are in their menstruation. This is typically referred to as the "loss of estrus."

If you mean "why" as in "what is the mechanism", then most things I've read indicate that it's caused by olfactory cues (smells in other words) causing the hormone systems of co-habiting women to synchronise.

If you mean "why" as in "what is the reason" then I am not sure that you can be quite so anthropomorphic as you have been - it may not confer any evolutionary advantage to all menstruate together, but also, it doesn't confer any evolutionary DISadvantage.

When we ascribe reasons to menstural issues, we often forget (as modern people with full control of our fertility) that in paleolithic times at the dawn of humanity as a species, most fertile women could well have been bred as often as possible, and that therefore women may have had many fewer periods in their life. This means that you have to be very careful in projecting reasons for issues relating to the timing of periods.