Metro Etiquette

The days grow warmer here in Santiago.
The metro rides grow sweatier.
And questions of metro etiquette go unanswered.
Por ejemplo, when do you give up your seat?

I live at the last stop on the red line of the metro, so when I board in the morning, I am almost always assured a seat. This would be just dandy, except that immediately after I get on the metro, so do jillions of other people. And some of those other people need a seat.

From my observations, if you are old, someone will give you their seat. If you are pregnant (or may become pregnant?!), you get a seat. If you have a screaming bundle of joyous mush, you get a seat. If you are on crutches, you get a seat. If you are missing toes, you probably get a seat.

But honestly, the seat-getters are not clearly catergorized.

So that lady on my left looks pregnant, but she could just be fat.
Do I give up my seat?
If she's not pregnant, would she beat me up?
That lady on my right looks kind of old.
Will she be offended if I ask to see her AARP card?
The man standing in front of me is way too close.
Should I give up my seat as a way of escaping his intrusiveness?

By the time I am three stops away from my house, I don't even want a seat anymore.
I certainly don't need one, unless you classify lack of balance during starts/stops as a "need."
And so I stand to avoid the awkwardness of seat-exchange and negotiation.

Speaking of awkwardness, armpit-face encounters plague the metro.
I have been considering growing out my own armpit hair for metro self-defense.
It it amounts to anything, I'll take pictures.
Promise.