This is a leftover from the caveman days, and a phenomenon exploited by Reality TV when they have 12 females battling over one male. Back when survival of the fittest actually counted for something, the girls in the cave would naturally be competing for the 'affection' of the Alpha Males in the room. Now, if you happened to have your monthly, that knocked you out of the running and gave your sisters better odds.

However, if you were in sync with each other, you all stood an equal chance. Another advantage to this can be found in other pack animals (cause face it, that's what we are). If we're all 'in heat' at the same time, we're all giving birth at the same time, at least in theory, and the pack functions more efficiently. It would be chaos to have babies dropping at random times when the tribe isn't prepared.

Of course, this isn't quite how it works anymore (with the exception of the aforementioned Reality TV), but like wisdom teeth, it's a physical indication of our pre-dawn history.

~Arlie