Reindeer are the past masters at keeping warm. When you evolve to live in temperatures below minus 30 Celsius then you need all your wits about you to keep warm.
To begin with they have an extremely dense winter coat, 2,000 hairs to the square centimetre although I have to confess to not having confirmed that by counting them myself! Secondly, each individual hair is hollow for the same reason that we have holofil in our duvets. Air is a very good insulator so the combination of air in each hair and around each hair increases the insulation factor. In fact, reindeer are like a mobile thermos flask, neither allowing the cold in or indeed the warmth out. A bed of snow is a comfortable spot for a reindeer, and they can lie on it without even melting it.
You may have noticed that cows or horses out in fields in windy weather tend to put their backs to the prevailing winds (as well as look pretty miserable). That is actually not a very clever thing to do, as the wind lifts the hairs and takes away more of their body heat. They need to copy reindeer who face a blizzard. By doing this they keep the hair across their body flat and so do not lose heat. The only disadvantage to this is you end up with a ice pack on your face. Lucky that reindeer have hairy foreheads.
Actually if you study a reindeer closely from nose to tail you will find no bare skin anywhere and that even applies to the bottom of their feet. Yet another fine adaption to the cold, and with the added bonus of improving their grip on the ice and snow.
Finally, if you measure the temperature of the blood of a reindeer at its extremities you will find it is cooler. Once again this is to prevent heat loss. To do this a counter current system has evolved whereby the warm blood exiting from the main part of the body passes close to the colder blood coming back from the extremities. The net effect is the cold blood is warmed and the warm blood is cooled and the heat remains in the body.
There are other heat saving aspects to reindeer but I think that’s enough for now. Food for thought however it does mean that they can get awfully hot in the warmer weather. I reckon reindeer would be the first to sign up to a programme to slow down global warming!