On a cold winter day in January, I found myself taking pictures around Swan Lake Sanctuary on Vancouver Island. The lake was frozen solid, a rare occasion. Despite the cold, ducks and seagulls waddled happily over the ice, gobbling up oaks provided by friendly visitors, and some rested on one foot or two, feathers fluffed up for warmth in the middle of the lake. Even in my warm winter boots, my own feet were feeling icy cold. So what about these ducks? Didn’t they have cold feet? And it turns out: No, they didn’t. Or maybe not as much.
Rete Mirabile, the “Wonderful Web”
Apparently, ducks, along with other birds including seagulls and flamingos, benefit from an interwoven network of arteries and veins in their legs and feet called the rete mirabile, the “wonderful web”. In the body’s blood circulation system, arteries carry oxygenated blood away from the heart to the body and veins transport oxygen-poor blood from the body to the heart. Through the close connection of artery and veins in the ducks’ rete mirabile, the cold blood in the veins coming from the feet cools down the warm blood in the arteries coming from the heart and body. As a result, cold blood circulates through the duck’s feet and because of the small temperature difference between their feet and the lake, they lose only small amounts of warmth through their feet. In fact, the amount of lost warmth is less than 5%.
This system is called counter-current heat exchange and aids many animals in regulating body temperatue.
Whales have the rete mirabile in their tails and fins and camels in their hump. In 2015 the first warm-bodied fish was discovered. The Opah, or moonfish, has a rete mirabile in its gill tissue structure.
In flamingos, the rete mirabile is thought to cool down body heat. Flamingos spend a long time standing in water that can warm up extensively during the day. Their body heats up in the sun and the cold blood coming from the feet is aids in cooling down their body temperature to avoid over-heating.
Counter-Current Heat Exchange
As an example of a biomimicry design, many Heating, ventilation and air conditioning (HVAC) systems for buildings use the same principle of countercurrent heat exchange. The air intake and output pipes run adjacent to each other in opposite directions. As outside air is brought into the building, it is made warmer or cooler by the building-temperature air flowing out of the ventilation system.
So now, every time I turn up the heat, I keep thinking of cold duck feet on ice. And every time I put on my boots in winter, I wish for a wonderful web to keep my feet warm.