So you think your feet are cold?

The next time you poke your one of your feet outside your nice warm bed in the morning and think it’s too cold to get up, just imagine being a reindeer, stuck outside, in the snow and ice, desperately looking for food in temperatures as low as -30°C.  There are a lot of people that don’t believe reindeer can fly, but to be honest, I find it pretty amazing they survive at all.

Cold Comfort

There are two problems with being cold.  Firstly, chemical reactions happen more slowly, and some even stop, at low temperatures.  Secondly, when water crystallises into a solid (freezes), it expands.

Because we humans are clever, and can paint, write poetry, even play the trombone (should the urge take us), we sometimes forget that when it comes down to it, all living things (including us) are, at the most basic level, chemical reactors.  So, when the cold slows chemical reactions down, it slows life down!

Living cells can’t survive freezing.  This is because they contain a lot of water, if that water freezes it rips the cell apart, killing it.  Have you ever seen what happens to a lettuce if it freezes accidentally in your fridge and then defrosts? It just turns to mush.  There are plenty of organisms that survive in temperatures where all the water around them is freezing, but they are only able to do this because they prevent ice-crystals forming within their cells in the first place.


Every winter, car owners add chemicals to the water in their car engines to lower the freezing point and prevent the water inside wrecking their engine.  A number of animals use a similar trick: the arctic ice fish, Trematomus has glygoprotein antifreeze in its blood.  The parasitic wasp, Brachon cephi is just one of a large number of insects, mites and other arthropods whose blood contains glycerol.  The larvae of the midge Chironomus have a neat trick, its cells are surrounded by dilute fluid and the cytoplasm within its cells is packed with sugars and salts.  This ensures that the water outside the cells freezes first, protecting the cells themselves.  The fluid in their cells can stay liquid at temperatures as low as -32°C.  Some animals, most notably deep living fjord fish are able to become “super-cooled” without freezing.

Mammals and birds have an inbuilt central heating system that keeps their core body temperature at around 37°C (40°C for birds).  To generate heat they break up chemicals obtained from their food or fat from their body’s stores.  The heat is generated in muscles by fast contractions (shivering), in the liver and in the fat cells themselves.  It is then distributed around their bodies by their blood as it circulates.  This extra heat frees them from the effects of cold weather, but it diverts both raw materials and energy from other essential life processes like growth, repair, muscle action, digestion etc.

Keeping your body hotter than the environment requires a constant use of energy and mammals can use up to 80% of their food in this way.  This is because heat always flows from hot to cold.  The cup of tea that I made twenty minutes ago and forgot to drink has gone cold because its heat has flowed away.  The room has become ever so slightly warmer as a result but sadly, the cup of tea was small compared to the room, so I haven’t noticed the room get warmer, just the tea becoming undrinkable.  Excuse me whilst I put the kettle on again.

Right, I’ve made not one, but two cups of tea.  One is in a plastic cup; one is in an empty baked bean tin (more of them later).

Trying to find the extra energy that heat generation requires, in winter, when food is at its most scarce presents huge problems.  For this reason, rather than deal with the cold, many animals opt to hibernate through it.  They allow their bodies to cool and slow down to the point where they are barely alive and remain inactive throughout the winter.  This way, their fat reserves last a lot longer and is perfect for animals like hedgehogs and shrews that feed on insects, worms, and other critters that  are either dead or dormant during winter.  Reindeer are vegetarians so there is always food available if they can get to it.  They have large hooves that are useful for kicking through the snow to find food.  Polar bears actually hunt other cold weather animals (including humans) so their available food actually goes up during the winter, for them, the lean period when they have trouble getting enough food is the Summer.

Happy Hibernators

Hedgehogs in Britain can be seen out and about as late as December if the winter is mild enough.  Often, these will be young animals that didn’t manage to get enough food during the summer.  If you see a hedgehog at this time of year, it needs help.  Put out tinned cat or dog food with added crunchy munchies (for their teeth), and water.  Don’t give hedgehogs bread and milk.  To survive winter, a hedgehog has to weigh approximately 450g by November.

Champion Hibernator

Hibernating animals normally burn just enough fuel to keep their bodies above freezing 0°C.  Not so the Arctic Ground Squirrel.  These super-cooled, snowbound, critters allow their temperature to drop as low as minus 2.9°C.

An animal can’t afford to generate unlimited amounts of heat, so to survive, it needs to limit the amount of heat it loses to the environment.  Remember, my two cups of tea?  The one in the metal tin was almost too hot to touch when I first filled it, but the tea inside is already going cold.  The one in the plastic cup was cool to the touch even when the water inside was boiling hot.  It’s warming up slightly now, but the tea inside is still beautifully hot and drinkable.  This marked difference between the two “cups” is due to their different heat conducting properties.  Plastic is a good thermal insulator; metal is a good thermal conductor.  This is why mammals have fur and birds have feathers, both provide excellent thermal insulation.  A good layer of fat or blubber is also helpful.  More than a food source, it’s also a great insulator.  Heat can also be lost when air, warmed in the body, is breathed out again.  Penguins, Reindeer, Polar bears and a number of other animals solve this problem by reclaiming heat and moisture from the air on its way out thanks to the rather special shape of their nasal passages.

Furry-Nuff (Get Down)

Both polar bears and reindeer have incredibly dense, two layered, coats consisting of a dense under-fur combined with an outer layer of long, hollow guard hairs.  The hairs are packed together extremely tightly; 100 guard hairs cm-2 and 200 woolly under-hairs cm-2.   The hairs trap air, which then insulates them so well that both reindeer and polar bears can lie on snow without melting it.  When you stroke a polar bear (only to be done when they’re anaesthetised or they’ll eat you!), they aren’t warm to the touch the way a dog or cat is.  The hollow guard hairs also help provide buoyancy when swimming.   Some books and websites will tell you that Polar Bear fur helps to collect and conduct sunlight to the bears skin, this is one of those famously wrong “facts” that has been around so long everyone believes it.  The most amazing bird feathers for providing warmth are the down feathers of the Eider duck (which we use to make Eider-down sleeping bags) and the feathers of the emperor penguin, the only animal to spend winter on the Antarctic Ice shelf where the temperature regularly drops below -40°C.  The funniest place you’ll find feathers is on the feet of the Ptarmigan.  This arctic relative of the grouse is the only bird in the world to have feathers growing on its feet.

Loverly Blubbery

Polar Bears have a layer of fat 11cm thick.  That’s a lot of blubber, but not as much as a right whale which can have a fat layer over 50cm thick making up around 45% of its body weight.  Blubber is especially important to animals in water because heat is conducted through water 27 times faster than in air.  Fur isn’t much use in water because can’t hold on to the air that allows it to insulate so well on land.  Whales don’t have fur but don’t spend time on land if they can help it.

Having a large body can also help an animal retain heat.  Guess what? You can illustrate how this works with a cup of tea as well.  Make enough tea to fill a bucket, measure its temperature and then put it in the garden.  Now fill a cup with tea from the bucket and leave it outside as well.  Check the temperature of both every minute.  The cup will cool down much faster than the bucket.  To grasp what’s going on you need to do a bit of maths: imagine a cube, where every side measures 2cm.  It will have a surface area of 6*(2*2)=24cm2  (because a cube has six sides), a volume of (2*2)*2=8cm3, and a surface area to volume ratio of 3:1.  If you increase the length of the sides to 20cm (ten times bigger), the surface area becomes 2400cm2 and the volume becomes 8000cm3, a ratio of 0.3:1.  An animal will lose heat across its surface to the cold outside, so, the less surface area it has relative to its volume, the less heat it will lose.  Thanks to their small size, shrews needs to eat the equivalent of their half their own bodyweight each day or die – even in the summer.

There is one part of the reindeer’s body that can’t be bulky; it’s legs.  Legs have to be relatively thin or walking is difficult and running is impossible.  This means that they have a relatively large surface area compared to their volume and can lose heat easily.  Heat loss from legs is a problem not only for reindeer, but for all animals in the cold.  Birds help cut down the heat lost through their legs and feet by tucking one leg at a time into their insulating feathers.  This option isn’t really practical for reindeer, polar bears or even penguins (if they’re balancing chicks on their feet).

Any blood circulating through an animal’s the legs will lose its heat.  When it returns to the core of the body, it will then take heat back in, lowering the animal’s overall temperature.  Cold weather birds and mammals avoid this by having a blood circulation that incorporates a “counter-current heat exchange system”.  Heat is taken away from the arterial (outgoing) blood before it goes into the legs and given back to the venal (returning) blood on its way into the body core.  That’s the heat exchange part, but what about the counter-current?  Well, the blood is running in opposite directions, so the coldest blood returning from the legs meets blood coming from the body that has already lost a lot of its heat.  This is the most efficient system because heat flows from hot to cold at a rate inversely proportional to the temperature difference, in other words, the bigger the temperature difference, the faster the rate of heat transfer.  If you want to check this out, get yourself a thermometer and a fresh cup of tea.  Now, stir the tea gently with the thermometer and note down the time and the temperature.  Wait 30 seconds and do it again.  You can draw a graph of the temperature (y-axis) against time from the first measurement (x-axis) to show the exact nature of the relationship.  Thanks to their heat exchange system, reindeer can have feet that are up to 30°C colder than the rest of their body.  If your feet got that cold you would be in quite a lot of discomfort.

Are you horrified at what a spoilt life we humans live now?  If you’re reading this in bed, shame on you.  Come on, get up, get some warm clothes on and go and feed some birds.  Remember, fat is best, so stick out some nuts and lard.  Oh, and don’t forget water too.

Have a great Christmas and I’ll see you in the New Year.  By the way, although I love Christmas cards of reindeer and can even believe they can fly (when I’m in the right frame of mind), I firmly draw the line at those pictures of polar bears and penguins on the same iceberg.  Like, that could really happen.  Duh.