Mammals and Temperature Ranges - Arctic Foxes

Factual Questions
1

I was reading an article on the Arctic-Fox where it stated that the animal regularly survives temperatures down to -50 degrees Celsius in the wild and has managed temperatures down to - 80 degrees Celsius in captive tests.

My first question is how something the size of cat can survive such insanely low temperatures, at least a polar bear has a large body mass to retain heat inside.

Secondly during the summer months the foxes also experience temperatures significently above freezing this means they have a temperature range with around 60 degrees variation over the course of a year. Is that the widest temperature variation of any mammal?

Thirdly I wonder what the maximum temperature is before the fox begins to appear in obvious discomfort?

Thanks!

2

Wikipedia says

3

Here in Montana, winter temperatures of 20 (F) below are common, 40 below happens from time to time. In the northern parts of the state, 50 below is not uncommon.

Yet mice, voles, rabbits, hares, skunks, raccoons, red foxes, grey foxes, ferrets, porcupines, and other small to medium mammals manage to survive the winters. Just digging a hole in the snow and curling up in it can make a huge difference. If the hole is on the leeward side of a drift (or tree, or hill, or whatever) and there’s a curve or dogleg in the entrance, body heat can be enough to raise the temperature of a small sleeping chamber dramatically.