Like water off a duck’s back

It’s obvious where that expression, “like water off a duck’s back” came from.  Duck feathers shed water amazingly well — their plumage seems almost impenetrable.

Droplets of water bead up and slough (or sluff) off the outer feathers of duck plumage.  This guy had just righted himself from tipping up to scrounge algae off the rocks at the bottom of the creek bed.

No doubt part of staying warm in the chill winter temperatures and winds is staying dry, and duck plumage is intended to do just that.  Not only are the feathers incredibly dense, laid down in overlapping layers in feather tracts, but they are coated with a waxy residue from a gland at the base of the ducks tail that waterproofs them.

Beads of water are shed from all surfaces from head to tail end on this hen Mallard.

But what about those bare feet, exposed to near freezing water temperatures and standing on cold rocks or ice or snow for hours on end?  Feet don’t shed water, just the feathers.

Cold toes?

This drake has just climbed out of the water, and is standing on ice, not something we would be comfortable doing (barefoot).  What happens when we reach for ice cubes in the freezer with wet fingers? The ice sticks to our fingers and is difficult to remove without losing some skin in the process. So how do ducks keep their wet feet from sticking to the ice?

Ducks slip and slide on ice, but their feet don’t stick to the surface.

The secret is to maintain very cold toes that are the same temperature as the surface on which the duck stands or walks.  This is achieved by having arterial blood going to the foot run in parallel with the vein that is bringing cold blood back from the foot — making a heat exchange unit that promotes cooling the extremities while preserving the warmth of the body core.  Engineers have used this principle in the design of heaters and air conditioners, among many other uses.

And this makes it nice for ducks to stand around admiring their reflections in icy pools.

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.