I saw a news story today on a bacterium that can withstand very high radiation exposure, freezing cold, & exposure to vacuum. Cool stuff. Said bacterium isn't alone in this, mind you, as I know from my colleague Allan Green that lichens have had much the same treatment, shot up into space & reviving once in more congenial conditions.
A few other organisms are capable of similar feats: tardigrades ('water bears') can survive losing up to 90% of their water content & while in this desiccated state can be frozen, plonked into ether, all sorts of nasty things, only to spring back into action once rehydrated. 'Higher' animals can't cope with such extremes, but some can still survive conditions that would test most eukaryotes.
Take arctic foxes. Like other animals living above the Arctic circle, these little foxes (they weigh only 3-4kg) must survive in the polar winter, where temperatures get down below -20oC - exacerbated by wind chill - for lengthy periods of time. In such conditions there's a big gradient between the animal's core body temperature of 37oC and the external environment, which you'd expect to result in considerable heat loss (Prestrud, 1991). Prestrud points out that, in order to maintain that core body temperature, the rate at which an Arctic fox - or other polar animal - loses heat must be the same as the rate of heat generation. And since generating heat requires an energy source, this places further pressure on polar animals as there's often not a lot to eat at this time of year. So increasing heat production isn't really going to be an option. The alternatives are reducing thermal conductance (a measure of how readily the body' exterior covering conducts heat to the outside) or reduicing the temperature gradient. You'd also expect to see behaviours that reduce energy expenditure, such as reduced locomotion, & predict that some way of storing energy e.g. as body fat.
So, how do Arctic foxes manage to survive during the polar winter? It seems that their basal metabolic rate, which gives an indication of heat generation, doesn't change between summer & winter. In addition, they're less active during the winter, so they don't appear to be coping by increasing heat output.
Their rate of heat loss is also seasonally constant (Prestrud 1991), something that's achieved via an increase in fur insulation of nearly 200% during winter coupled with a slight decrease in skin temperature. (This is probably the result of vasoconstriction, where blood vessels in the skin constrict, reducing the flow of blood & consequently of heat loss.) Interestingly there's a lot of variation in the thickness of fur over the fox's body. In winter it's thickest on the feet, backs of the legs, & back & sides of the body - the parts of the body that are most in contact with the snow (either when walking or when lying down), In fact, the pads of their feet are actually covered by fur, which makes sense when you consider that the foxes are walking on surfaces well below zero - without some form of protection, & probably also good control of blood flow to the foot pads, their little feet would freeze solid. (Blood flowing to their feet is probably cooled by acounter-current heat exchange system, where arteries carrying warm blood out to the feet pass close to veins where blood flows back to the body. Heat is lost to the venous blood, so that the arterial blood is relatively cool when it gets out to the animal's toes.)
It's as well that their fur is such a good insulator, because the foxes' small size places them at a real disadvantage in such cold conditions. The smaller you are, the greater your surface area: volume ratio, which means that smaller animals will lose heat faster than larger ones. (And the fur can't be too long, either, or it'll get in their way as they move around.) To some degree this effect is lessened by the fact that Arctic foxes have small ears, short muzzles, short legs relative to body size, and rounded little bodies (which has the effect of making them almost irresistibly cute to human eyes. You see the same thing in puppies & kittens, & some degree we've probably selected for retention of these cute, neotenic characters in adult domestic pets.) Curling up into a ball when at rest also helps reduce heat loss, as the ball shape presents the smallest possible surface area to the external environment.
But they can't stay that way all the time - a fox has to eat. There is food available during the polar winter: lemmings, ptarmigans, hares, & seal pups. The foxes scavenge as well, feeding of carcases of animals such as musk oxen. Food caches may help, provided you can remember where they are, or the food isn't pillaged by someone else. But Prestud comments that starvation is still a significant cause of death during the winter, especially if stormy conditions prevent foraging for more than a few days. Arctic foxes really do live life on a knife-edge.
P.Prestrud (1991) Adaptations by the Arctic fox (Alopex lagopus) to the polar winter. Arctic 44(2):132-136