What Do Hibernation and Shivering While Urinating Have in Common? [Reading Science]

These are the cubs of the Jirisan half-moon bears hibernating. It is said that they visit during hibernation to check on their health.
[Photo by YouTube screenshot]

[Asia Economy Reporter Kim Jonghwa] Although less common nowadays, in the past, most people who urinated outside on cold winter days would shiver uncontrollably. Dogs also sometimes shake their bodies after urinating to mark their territory. Why is that? It is said that when urine is expelled from the body, it carries away body heat equivalent to the amount of urine, causing the body temperature to drop momentarily by about 1℃.

Humans generate heat by shivering to raise the lowered body temperature. Maintaining body temperature is very important for both humans and animals. Whether human or animal, maintaining body temperature during winter when temperatures drop below freezing can be a matter of life and death.

One of the methods animals choose to maintain body temperature, in other words, to survive the cold winter, is 'hibernation.' In winter, the weather is cold and it is difficult to find prey. Animals that cannot find food during winter choose hibernation to minimize energy consumption.

Animals that hibernate can be broadly divided into two types based on whether they can regulate their own body temperature: endotherms and ectotherms. Endotherms maintain a constant body temperature regardless of the external temperature. Bears, squirrels, raccoons, and hedgehogs are examples of endotherms.

Ectotherms are animals whose body temperature changes according to the external temperature. Frogs and snakes are representative ectotherms. Hibernation is essential for ectotherms. Since their body temperature can drop below 0℃ and they can freeze to death, ectotherms enter a state of hibernation that resembles death until the weather warms up in spring. Their breathing and heartbeat are almost nonexistent during this time.

In contrast, the hibernation of endotherms is considered a light sleep compared to the death-like sleep of ectotherms. Endotherms have a high basal metabolic rate to maintain body temperature, so before hibernating, they eat enough food to produce energy and during sleep, they maintain minimal breathing and heartbeat.

Unlike regular sleep, hibernation in animals is similar to a coma-like state. Therefore, after waking from hibernation, a certain rest period is necessary.

The bear is a representative animal that hibernates. As an endotherm, the bear spends winter in a shallow sleep state in a den made of wood or rocks, consuming fat stored in autumn. Bears sometimes wake up during hibernation to defecate or eat. Their goal is to minimize movement during winter to reduce energy consumption.

In the case of squirrels, they eat a lot in autumn to build up fat layers and then make nests in thick leaves or well-insulated underground places to sleep. When squirrels are active, their heart rate is about 150 beats per minute, but during hibernation, it drops drastically to about 5 beats per minute.

Like bears, endotherms such as squirrels occasionally wake up during hibernation to eat food stored in their nests. However, the hibernation of ectotherms like frogs and snakes involves almost complete suspension of bodily functions. In frogs, a substance called fibrinogen acts like antifreeze, preventing the body from freezing and allowing minimal life support.

Animals that do not hibernate include tigers, deer, foxes, rabbits, and flying squirrels. Tigers, deer, and foxes grow thicker fur before winter to withstand the cold well. Rabbits also change their fur to white and grow thick fur in winter, making it easier to survive. Flying squirrels store acorns and other food in autumn to use as winter food, so they do not face major problems during winter.

Animals living in tropical regions where food is abundant year-round, and animals like bears in zoos that always have access to food, do not need to hibernate.

A cub sleeping in its mother bear's arms during hibernation.
[Photo by National Geographic TV screen capture]

So, what is the mechanism that induces animals to hibernate when winter comes? It is said that animals that hibernate have a protein in their blood called 'Hibernation Inducing Trigger (HIT).' When days get shorter, temperatures change, and food becomes scarce, this HIT triggers hibernation.

There is also another type of protein called 'Hibernation-specific Protein (HP).' In some animals possessing HP, when they are not hibernating, HP exists in the blood at about 60-70μg per 1ml, but during hibernation, it decreases to about 1/10 to 1/20 of the normal level. Conversely, when the blood HP level returns to normal, the animal wakes from hibernation.

A hormone called 'enkephalin,' secreted by the animal's brain, is also known to induce hibernation. Scientists are researching 'artificial hibernation' for humans using substances like HIT, HP, and enkephalin.

Lowering a patient's body temperature to 18 degrees Celsius stops brain activity and blood flow. If hypothermic surgery can be performed in this state through artificial hibernation, organ transplants and surgeries can be done without losing a single drop of blood. It is also used in cancer treatment by putting normal cells into hibernation before chemotherapy, allowing targeted attacks on active cancer cells, among other applications.

The secrets of animal hibernation have not yet been fully uncovered. It is hoped that as the mysteries of hibernation are gradually revealed, they will also aid in human disease treatment. Could the commonality between humans shivering while urinating and animals hibernating be that both are behaviors to maintain body temperature?