Don’t say “freezing like a dog” when the temperature drops. This is a bit unfair to dogs…

As the weather in the northern hemisphere gets colder, humans, who evolved in hot climates, are beginning to complain. Animals living in the Arctic and Antarctic often have to endure temperatures as low as -40 degrees Celsius. They naturally need special ways to keep warm.

The simplest way is of course to wear more clothes . The effectiveness of animal fur in keeping out the cold mainly depends on the thermal conductivity of the hair itself and the air between the hairs. Most furry animals have two layers of hair, coarse guard hair and fine inner down, which can "catch" as much air as possible. There are also many small holes in the guard hair of reindeer, which store air like insulating foam plastic, further increasing the warmth effect.

Although people in Northeast China believe in wearing mink, fur still has its limitations. The first is that the fur of small animals cannot be too long. Although the stoat lives in the polar regions, its fur is not particularly effective in keeping out the cold. Because it relies on flexibility to hunt, it can only have short fur. If the fur is too long, it will be like a child wearing a down jacket, becoming a "big bun" with difficulty in moving.

Second, the "thermal underwear" composed of fluff and air will be less effective when it gets wet. Seals and whales rely on another "underwear" - the fat layer. The fat layer is not as good as hair in keeping warm, but it is not afraid of getting wet and can store energy for survival. The fat layer of the bowhead whale living in the Arctic can be up to 30 cm thick, making it the world's fattest animal.

Even furry animals on land have weaknesses that force them to be exposed. The paw pads of Arctic wolves and Arctic foxes are hairless and located at the end of their limbs, farthest from the body where heat is generated. The skin on their paws has a highly developed network of blood vessels that heat the soles of their feet like a heater. The strange thing is that they do not adjust the temperature of their paws to the same as their body temperature, but keep it around 0°C. When the temperature of humans' hands reaches 15°C, they are already very cold, but the descendants of wolves - dogs, can walk normally when the temperature of their paws is 0°C. So it is unfair to dogs to be "frozen like a dog".

Another critical part is the respiratory system . Although the lungs are hidden in the body, they have to be exposed to the cold and dry air outside. The nasal cavity of reindeer is filled with thin sheets of tissue. In order to increase the surface area, it is curled up and twisted like a flower roll. After the air is inhaled, it is heated by the warm tissue and humidified by the evaporation of the nasal mucosa, so it can be inhaled safely without hurting the lungs.

All of the above methods are to achieve the effect of heating without increasing energy consumption as much as possible. Energy (food) is not easy to find in the North and South Poles, but when forced to do so, animals will sacrifice energy in exchange for temperature to save their lives. This strategy is especially common in young animals, because they are small and heat is lost more easily, and their cold-proof sweaters are often not as thick as those of adult animals.

The simplest way to keep warm is muscle trembling , which is what we commonly call "shivering from the cold." Interestingly, ptarmigan chicks keep warm by trembling their pectoral muscles, because the pectoral muscles are the largest muscles in a flying bird and produce considerable heat.

A more durable way to keep out the cold is brown adipose tissue. Although it is called adipose tissue, it does not contain much fat. It can quickly break down fat (the fat it uses mainly comes from white adipose tissue, which is ordinary fat), and more than 90% of the energy generated is converted into heat energy. Therefore, brown adipose tissue is the stove of animals and can save lives at critical moments. Although young reindeer, musk oxen and harp seals are wearing thick sweaters, they still need brown adipose tissue to generate heat to maintain body temperature. The energy consumed by the brown adipose tissue of a young reindeer can account for 70% of the energy consumed by its entire metabolism.

Author: Red Queen Popular Science Creator

Review丨Meng Xianlin Professor-level Senior Engineer of China Wildlife Conservation Association

Huang Chengming, Professor of Hainan University

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