Just realise some interesting girl group behaviour pattern few days ago, though nothing spectacular, but noteworthy (this is not Discovery Channel, ok!).
Girls always stick together whenever they go out and they won't be willing to be separated even for the shortest time period. When they are travel out, all of them must stick together in the same car. If you have to squeeze 6 of them in a car, they will squeeze, even when there is another car with no females in it. Don't bother trying to separate them, as they will ignore you or show you a black face if you apply enough force. While in the car, they will make as much noise as possible, and pity the driver if it is a he (like Kenny describe it, is like a 5.1 surround sound system). A guy can be happy among a group a girls; but a girl might not be happy around a group of guys. Of course, nothing is absolute and they are bound to be exceptions, just talking stereotypically.
Come to think of it, I think I observed this trend quite a few times, when travelling, when dining, when sitting for conference and etc. Guys usually don't quite mind who they sit with, as long as they feel comfortable with the next person. Girls on the other hand, are more picky with who they sits with. Usually they prefer their usual partner, probably they need to chitchat to kill time (Guys are too boring and primitive, can't fulfilled their chitchatting need). If they can't find any of their preferred sitting partners, maybe they might preferred to sit alone and suddenly became very quiet (Then you would notice them as you are not used to the sudden peace).
Conclusion? Girl need to flock together to fulfilled their chitchatting need (and perhaps to avoid loneliness, even temporary ones). Probably it is written within their genes. What guys do then? Hmm, need to wait for another male observation blog from a female counterpart.