Cold Feet

On one of the blogs I read, we got into a discussion of how ducks can stand on ice without sticking to it or freezing their toes.

Mallard walking on ice  (photo by Mike Powell).

This happens to be a favorite topic from my former physiology course, and deserves more discussion here.

Although many of us are walking around with cold feet (despite warm boots) these days, we humans would definitely stick to the ice if we ventured out barefoot.  Exposing human toe (or finger) flesh to sub-freezing temperatures can result in severe frost bite and eventual loss of the appendage.  So how do animals manage?

By utilizing a clever adaptation to conserve body heat in extreme cold weather called counter-current heat exchange.  The counter-current part means there are two opposing flows; the heat exchange part means that the two opposing flows exchange heat across their length.  For example, a pipe with warm air gives up its heat to a pipe with cool air coming from the opposite direction.  The end result is that the two pipes are the same temperature at each end, i.e., warm at one end, cool at the other.  Now applying this rule to the problem of ducks walking on ice…

With no CC exchange (left), the foot stays warm and loses heat to the ice.  A warm web would also stick to the ice!  With CC exchange (right), arterial blood going to the foot is gradually cooled by giving up its heat to the vein bringing blood back from the foot.  The foot temperature is thus about the same temperature as the ice it is resting on.  Less heat loss!    Figures from (Ask a Naturalist)

Many animals use the counter-current heat exchange to conserve heat in cold weather.  Photo credit.

There is one other problem, however, which we have all experienced.  Cold extremities with low blood flow to them tend to be numb and don’t work well, causing us to be clumsy.  Now this would be disastrous if you needed your extremities to catch your food and they didn’t work.

The Arctic Fox trots across ice and snow in Alaska, never worrying about having numb toes that can’t hold on to its prey.  Photo credit.

So, one further adaptation is necessary:  modify the nerve conduction speed so that they work just as fast in the cold as they did in the heat.  That requires some changes in the fat composition of nerve membranes, but we don’t need to go into the biochemistry of that change here.  Animals are able to do this; humans are not.  We are really tropical animals displaced into cold weather for which we are poorly anatomically adapted.

Now here’s a challenge:

What if you had to drag this long, naked tail around on the ice?

I have never looked, but I bet the muskrat has two long centrally-located blood vessels in the middle of its tail that it uses as heat exchangers when exposed to cold.