By John G. Woods
As someone who loves the outdoors but struggles to keep his feet warm, I find the sight of mallards waddling around on the ice at the Salmon Arm wharf astonishing.
Why don’t their feet freeze? And, why doesn’t the heat lost from their feet lead to hypothermia?
Part of the answer to the “not freezing to death” problem is obvious. Ducks have a dense layer of down under their stiff outer feathers that insulates most of their body — ducks and geese are the inspiration for our down jackets! They also have several cold-weather behaviour tactics such as seeking shelter from cold winds and resting on the snow with their heads snuggled into their back feathers. This huddling reduces their body surface exposed to the cold, hence less loss of heat to the air.
But what about when a duck needs to stand or swim? Its bare feet are exposed to the elements and you would think that would lead to rapid heat loss, and ultimately, frozen feet. Here is where their superpower comes into the story.
If we could look inside those webbed-feet, we’d find an intricate system of blood vessels. Warm blood from the heart headed to the feet is closely routed through a network of veins carrying cooler blood from the feet. Because warmth transfers from hot to cold, the blood headed to the feet cools as it warms the cold blood heading back towards the duck’s heart. The greater the difference in temperature, the more rapid the transfer. The end result is cold, but not frozen feet, and a body core temperature that remains relatively constant. Scientist’s call this elegant design a “countercurrent heat exchange.”
In one study of this countercurrent system in a gull’s foot, arterial blood at 35 C near the heart was cooled to 1 C by the time it reached the feet. Meanwhile, the blood leaving gull’s feet had warmed up to about 33 C by the time it returned to the bird’s heart.
In extreme conditions, ducks also can stand on one leg for prolonged periods — one foot on the ice, the other snuggled up into the warmth of their breast feathers. Presumably, they alternate feet like we shift our cold hands in and out of our pockets on cold days. I’ll need to watch for this.
As many walkers at the wharf know, we also have a few great blue herons overwintering along the foreshore. Their long-legs and huge feet make them another beneficiary of the heat exchange system found in many birds. Like ducks, herons rest by standing for long periods on one foot, with the other neatly tucked into the warmth of their belly feathers. Unlike ducks, herons don’t lay down on the snow to keep warm. Standing like statues, the herons make it through our long winter nights by tucking their heads and necks into their breast feathers, bundling themselves with their wings and depending on their countercurrent blood flow to keep their feet cool and their bodies warm.