Shirtless

I have been running track lately and have noticed something, even when its 50 degrees with a windchill of 30, the distance people run shirtless. Why? Is it because they want to show off their abs? Do they really get hot that quickly? Do they want to anger the ice-cream people who have a sign up for the ice cream vendors not to sell to shirtless people?
I have heard the girl track team making disgusted noises at them, so it probably not the girls. And given the fact they are hugging themselves for warmth, they aren't all that hot either, which only leaves the ice cream man solution.