In the middle of winter, how come wild ducks aren't afraid of getting their feet frozen when walking on the ice?

Wild ducks on the ice. Photo by Zhong Zhenyu

"There is a large group of wild ducks frozen on the river ice and unable to move" "Will they be frozen to death?"... In winter, enthusiastic citizens found these situations by the river and were very worried, so they immediately called the police for help. When the staff rushed to the scene as soon as possible, they found that the wild ducks were not frozen or frozen to death, and they were playing and fighting from time to time. Why do wild ducks always stay on the ice? Aren't they cold standing on the ice barefoot for a long time?

The ice surface becomes the best place to hide from natural enemies

In the winter in the north, the weather is freezing and the water surface is frozen. Water birds that rely on water for survival usually fly to the warm south for the winter when the water surface is frozen. However, there are also many wild ducks and other water birds that are "lazy" and stay in the north. They often stay on the wide water surface or ice surface. So in winter, we can always see groups of wild ducks lying on the ice in Beijing's parks, reservoirs or rivers.

Wild ducks generally belong to the Anatidae family of the order Anseriformes. They have dense feathers, flat beaks, and full webbed feet between their front toes. They are waterfowl and have adapted to a variety of water bodies. Swans, geese, and a variety of ducks are all members of the Anatidae family. In nature, they are prey in the food chain and need to be on guard against being hunted.

In winter, the temperature is low, and animals know that the grass, woods and caves are warm and are good places to keep warm from the wind and snow, but these places have long been occupied by predators. As prey, wild ducks have to choose the safer ice surface. In winter, as long as the water surface freezes, even if it is just a small piece of floating ice, wild ducks will crowd on it to spend the night. During the day, they basically move around on the ice and in the ice water. Generally, they only go to farmlands to forage in the early morning and evening, and it is rare to see them on the shore for a long time.

In terms of avoiding predators, there are indeed many advantages on the ice. For example, it can keep away from natural enemies such as land cats and weasels. These natural enemies have soft paw pads and cannot withstand the cold of ice and ice water, so they generally do not venture into them; wild ducks generally stay in the middle of the lake or river, with a wide field of vision, and can spot natural enemies at the first time, so they have enough time to fly into the sky or jump into the water.

The outer layer of the skin is wrapped in a "hard shell" to insulate and waterproof your feet from freezing

The reason why wild ducks are not afraid of the cold even though they are barefoot is related to their special anatomical structure.

The wild duck foot (the bare part without feathers) includes the tarsometatarsus (the sole of the foot), the toes and the webbed feet between the toes, which are mainly composed of bones, tendons and skin. The muscles extend here to become tendons to connect the joints. The tendons are mainly composed of hard and dense connective tissue. The skin is smooth and featherless. Because the wild duck foot lacks muscles and hair follicles, the metabolism rate is slow and it does not need more blood vessels and blood, which greatly reduces the heat loss caused by blood flow.

The outermost layer of the skin on the soles, toes and webbed feet of wild ducks is covered with a thick layer of hard yellow skin, which is the outermost layer of the skin, the stratum corneum, which is mainly composed of more than a dozen layers of flat dead cells without nuclei and organelles. The stratum corneum is very dense and has strong heat insulation, waterproof and insulation functions. This tailor-made "hard shell" can protect the bare feet of wild ducks from the cold outside.

The scaly protrusions and rough surface on the yellow skin increase the gap between the skin and the ice, reduce the contact area, and further enhance the heat insulation performance. This is actually an important reason why duck feet are not afraid of freezing.

Special blood circulation system to reduce heat loss

The reason why wild ducks are not afraid of freezing their feet is that they have a special blood circulation system called the "countercurrent heat exchange system", that is, the arteries that supply blood to the feet and the veins that return are intertwined. Through close contact, the warm arterial blood can directly transfer heat to the colder venous blood, effectively reducing the temperature of the blood flowing into the feet. Therefore, the temperature of its feet is also very low, only 1℃-3℃ above the freezing point, reducing the temperature difference with the ice surface, which can reduce heat loss and not feel frozen feet.

Long-term evolution, adapting to the environment from head to toe

In addition to the evolutionary adaptations of their body structure, wild ducks also change their behavior to prevent their feet from getting cold.

As a bird, the wild duck is covered with thick feathers, which retain air between the feathers, preventing heat from escaping and playing a role in insulation and warmth. In addition to the feathers and the air between them, there is also the skin and thick fat under the skin to maintain the perfect body temperature of 40℃.

Wild ducks often stand on one leg, alternating between the two legs, with one leg supporting the body to reduce the contact area with the ice, while the other leg is retracted into the thick feathers on the abdomen to keep warm. When lying down, the feathers on the body cover the feet to avoid losing more heat.

Through long-term evolution, the wild duck's incredible feet and special survival mechanism enable them to adapt to some harsh environments. Wild animals have their own survival rules. We should respect and protect wild animals and not interfere with their lives at will. However, once there is injury, lack of food or extreme conditions that threaten their lives, we should promptly seek help from relevant departments and take positive and effective measures to protect wild animals.

(The author Zhong Zhenyu is the deputy director of the Beijing Biodiversity Conservation Research Center)