You mean more insulated ladies don't you. As the ones with less fat, will already have warm bottoms. Actually, I'd be willing to help out with some sort of comparison test so we could see who's got the coldest bottoms, those with less or more insulation.

I'm sure there are pros and cons for each. You are right though a test is the only way.

Calling all ladies of all sizes and insulation level for a test form an orderly line facing away and grab your ankles please I'm coming!

As Richard mentioned. It's one of your body's extremeties. Your body will pump blood to parts of the body which need it the most ie the working muscles. As your bum is not a major working muscle when you run, it will get cold due to the lack of blood flow to that part of the body. Hope this helps answer your question. I don't have a bum factuation, just applying some science to the subject

The gluteus maximus ("bum muscles") are the largest muscles in the body, aren't they? I would imagine them to be working quite hard during the course of a run.

__________________
ONLY THOSE WHO WILL RISK GOING TOO FAR CAN POSSIBLY FIND OUT HOW FAR THEY CAN GO.

The glute muscles are used alot in running, but the buttocks do have a cushion of fat which gets cold due to a lack of blood flow to this area. Hope this helps explain it better.