Don't ducks have cold feet? Don't ducks get cold feet when they walk in the snow? Let's see if Science can help us unravel this mystery! Cold, the ducks? Never !
Outside, it's cold, a duck cold, to use the expression that refers to low temperatures faced by hunters during fall and winter. Yet if he is well an animal that does not suffer from low temperatures, it is him.
Looking at him a little more closely, one wonders about the contrast between his body protected from
cold by a thick down that covers a layer of fat and its bare and thin legs that most often remain submerged in ice water. How does he not catch the dead where such a foot bath could be fatal to humans?
This is thanks to an ingenious device with which it is equipped. “Indeed, explains the naturalist JeanMarc Fivat, in summer, the blood circulates uniformly in their body but during the season cold, blood circulation to the skin and extremities is limited. The vessels under the legs constrict, returning blood to the body. By a system exchange between veins and arteries, the arterial blood that goes down to the legs transmits, before of having reached them, its warmth to the venous blood which returns to the heart. The temperature is therefore strongly lowered in the legs but still remains a few degrees above zero thanks to vessels which dilate at regular intervals so that they are supplied with blood, oxygen and nutrients. (…)
Why then do thousands of ducks from the north travel so many kilometers to come to us and usually only gain a few degrees on the thermometer? "It is not the cold in itself that is at the origin of their arrival, explains the specialist, but the freezing water, especially the Baltic Sea, which prevents them from finding their food then trapped under the ice. Deprived of algae, shellfish and fish, they land on our large lakes generally untouched. These migrants therefore adopt a attitude of nomads during harsh winters before resuming in mid-February, the road to their regions of origin ". Mallard and goose are less dependent on water. Indeed, unlike the majority of water birds, geese do not find their food in lakes but in fields (grasses, wheat, corn). When the cold keeps the snow cover on the ground, then it is time for them to go further south. The case of the mallard is also special. This opportunist does not hesitate to go to the cornfields eat grains left on the ground or under oak trees to find acorns when algae, seeds, invertebrates and bread are scarce. Reason why the Swiss mallard usually stays in Switzerland all year round.
Another explanation comes from the popular belief that, in very cold weather, ducks can have their feet caught in the ice of bodies of water. The expression “it's freezing cold” could therefore be the contraction of “it's freezing cold the legs of a duck”. This expression comes from duck hunting, which is practiced in autumn, but also in winter where the hunter must remain motionless, on the lookout, and let the cold penetrate him to the bone while waiting for one of his poor victims. please come closer.
As the duck is chasing itself for a long enough period in a part of which it is not necessarily cold, it is necessary to specify what this 'duck cold' corresponds to. As everyone knows, the duck lives readily on lakes and ponds where it has its habits a large part of the year. The body of water protects it in part from its terrestrial predators of which humans are a part. When the temperature becomes very cold and the ponds freeze, the duck is forced to move and joins flowing waters, streams and rivers, less prone to frost, which increases its mobility and therefore its visibility. And it would indeed be on these very cold days that the hunter would have the best chance of meeting this poor palmiped.
The most obvious physical protection birds have to protect themselves from the cold is their insulating feather coat.
The feathers are inert and contain no skin or vessels blood, and thus does not disperse body heat into the environment. On the contrary, they retain body heat by trapping hot air near the skin. The number of feathers varies with the seasons, and are more numerous in winter than in summer. Besides the flight feathers of the wings and the feathers covering their bodies, waterfowl
have thick layers of down, especially on their chest and stomach. The downy feathers are specialized: they are small, light and their beards are not tangled. Their main function is to conserve heat. Down feathers are very abundant in certain species (ducks, geese, etc.); they are sometimes torn off by
the bird on its own body in order to fill the nest. Anyone who has ever worn a lined coat
of downy feathers knows how effective they are in winter and insulate well from the cold.
Subepidermal fat Under the feathers is the skin, which also plays an important role in controlling the loss. The skin is supplied with blood vessels which carry warm blood from the body to exterior surfaces. In addition, the dermis, the layer of skin located immediately below the thin outer layer of the skin (the epidermis), is a site of fat storage. The sub-epidermal storage of fat is particularly important in ducks that live in cold aquatic environments.
These fat reserves have two functions: they provide additional insulation by compared to feathers, and they serve as a reserve of energy that can be mobilized in cold weather when metabolic demands are high and food availability is low.
Reminder: metabolism and body temperature
Metabolism is the set of physical and chemical (enzymatic) reactions that take place produce within a living being and more specifically in its cells and environment interior (blood and lymph). The basic (or basal) metabolism in humans is the amount of heat released by a being not asleep, at rest physically (lying down) and intellectually, at an ambient temperature of 25 ° C and on an empty stomach for 12 hours (i.e. not performing digestive activity). Birds have a high basal metabolism and therefore need a lot of energy resources. Proportionately, large birds consume less energy per unit area than the little birds.
Along with metabolism, birds tend to have a temperature body higher than mammals. It is generally understood according to the species between 38 and 42 degrees. That of large wingless birds such as ostriches and some water birds like penguins have a lower body temperature than other birds. Finally, birds are generally larger in cold environments than in tropical areas, because they have a proportional body surface area lesser.
Birds living in cold environments must conserve their body heat to avoid hypothermia. However, the blood, which flows from the center of the body (heart) to the periphery (such as paws and feet) can easily lead to heat loss through dissipation in the atmosphere at the level of the skin. To prevent such a loss, birds have a real heat exchanger with counter-current made up of blood vessels (arteries and veins) located in the legs and which allows heat to be recovered and saved.
This principle of counter-current heat exchange is so efficient and ingenious that it has also been
adapted by humans to optimally heat buildings while limiting waste energy in winter! Bird's counter-current heat exchanger consists of arteries and veins adjacent and whose blood streams go in opposite directions. The artery brings blood hot in the paws, and the heat of the blood in the artery is transferred to the circulating blood in the vein (of course, oxygen and nutrients continue to nourish the cells of the paws). As a result of this heat exchange, the blood in the bird's legs is relatively fresh and little heat is lost. Thus, a duck standing on ice loses a small amount of heat through its paws.
While the fundamental body temperature of a duck standing on ice is around 37.8 ° C, the temperature of the bird's legs can be just above 0 ° C. To preserve their heat in cold weather, waterfowl also reduce the volume of blood that flows to their paws by constricting the blood vessels in their paws. Of experiments have shown that ducks gradually reduce the flow of blood to their legs
as the air temperature drops to 0 degrees. However, when the temperatures drop below 0 ° C, the birds re-increasing the blood flow to their legs to prevent tissue degradation.
Birds also protect their legs by tucking them into the feathers of their flanks and close of their body. To minimize exposure in cold weather, waterfowl should place themselves often on one paw, occasionally placing the other paw in the feathers of their body to protect it from the cold. A cold adaptation of the cells of the tissues of the legs In most birds, part of the legs are featherless. The longest part without feather is made up of the tarso-metatarsus (the foreleg bone). This bone corresponding to a fusion of the tarsus and metatarsus and is the equivalent of the bone in our foot. The tibia and the femur are generally covered with feathers. Thus, part of the paw and the fingers are not isolated. To keep their paws too hot than the rest of the body, birds should send a large amount of blood hot to these exposed areas. But on the contrary, they let their paws cool down in order to only have to provide them with a small amount of blood. In Herring Gulls, for example, the part of the leg exposed to the elements may have a temperature of only 4 ° C, and the below the leg is just above 0 ° C!
In order for the tissues of the lower leg to endure such low temperatures, their cells had to adapt. Each living cell is surrounded by a semi-permeable membrane consisting mainly of a bilayer of phospholipids.
Each phospholipid is made up of two hydrophobic chains of carbon atoms called fatty acids pointing towards the center of the membrane. To work suitably, the phospholipid of the membrane needs to be able to move in the plane of the membrane: it must therefore be sufficiently fluid to allow small molecules like oxygen to enter the cell and carbon dioxide to come out. The membrane can adopt different states depending on the temperature and pressure.
By example, a membrane of pure phospholipids forms a liquid-crystalline phase at 20 ° C and 1 bar pressure while at -20 ° C it rather forms a gel phase. So when the membrane is exposed to cold temperatures, it forms a more rigid assembly. Oxygen cannot then more diffuse in the cell, and the freezing often results in the death of this one. Each fatty acid carbon atom of a phospholipid has four attachment points.
Usually two of them serve to adhere to the two carbon atoms of the fatty acid adjacent. The other two attachment points usually attach to a hydrogen atom. However, it is possible that two adjacent carbon atoms lose one of their atoms. hydrogen and establish a double bond between them, forming trans fatty acids. Is the then calls unsaturated fatty acids (in hydrogen), i.e. comprising at least one
C = C double bond between two carbon atoms.
Unsaturated fatty acids tend to stay in a liquid state at room temperature. So, by creating double bonds between their carbon atoms, the fatty acids of the membrane cytoplasmic birds' feet allow the membrane to remain liquid even under cold temperatures, and so continue to operate normally. Why don't most birds have feathers on their legs? This is related to the fact that the metabolism of the bird is high to provide enough energy. For the flight. The excess additional heat produced must then be removed, even under colder climates, to avoid overheating. The featherless legs are the place where this heat is evacuated. the presence of feathers would prevent this evacuation.