Hibernation

Some animals sleep through the cold winter.

This is called hibernation. It helps them save energy. They do this when food is hard to find.

Before they sleep, many animals eat a lot. They store this food as fat.

This fat keeps them alive while they rest.

During the long sleep, their bodies change. Their heart beats very slowly. Their breathing also slows down. This helps them use very little energy.

Some animals, like bears, stay in a den.

They do not eat or drink. They live off their stored fat.

Hibernation can last for many months. It is a smart way to survive the winter.

Some animals sleep through the cold winter. This is called hibernation. It helps them save energy when food is hard to find.

To prepare, many animals must store energy. Large animals eat a lot of food. They store this energy as fat.

Small animals might hide food in many places instead.

During hibernation, an animal's body changes. Their heart beats slowly. Their breathing also slows down. This is called metabolic suppression. This means their bodies use very little power.

There are two main ways animals hibernate. Some are obligate hibernators. These animals sleep every year when the season changes. They do this even if food is easy to find. Others are facultative hibernators. These animals only sleep if they are cold or hungry.

Bears are special hibernators. They do not get as cold as small rodents. They can even stay in their dens while having babies.

They do not eat or drink. They use their stored fat to live. They even use special ways to keep their bones and muscles strong.

Hibernation is a special way some animals survive the winter. It is a state of very low activity. This is also called metabolic reduction. When animals hibernate, their body temperature drops. Their heart rate and breathing also slow down. This helps them save energy when food is hard to find.

This process is often called overwintering. Some animals use a similar method in the summer. This summer version is called aestivation.

To prepare for the long sleep, animals must save energy. Larger animals often become hyperphagic. This means they eat a huge amount of food. They turn this food into fat deposits in their bodies.

Smaller animals might use food caching instead. This means they hide food in different places to eat later. Some mammals even hibernate while they are pregnant. For example, female black bears hibernate to give birth. They gain extra weight to keep their newborns warm.

During the sleep, bears may lose 15% to 27% of their weight. They use their stored fat for energy.

Scientists study different types of hibernators. Obligate hibernators enter hibernation every year. They do this even if it is warm or food is plenty. Ground squirrels and European hedgehogs are obligate hibernators.

Their body temperature drops to near the temperature of the air. They sometimes wake up for short periods. This is called an arousal. Some think they do this to sleep better. Others think it helps them fix their immune system. Facultative hibernators are different. They only hibernate if they are cold or hungry. A chipmunk is a facultative hibernator. A white-tailed prairie dog is an obligate hibernator.

For a long time, people argued about bears. Some thought bears did not truly hibernate. This was because their body temperature only drops a little. However, research in 2011 and 2016 proved they are hibernators.

Bears can recycle their urine to stay hydrated. They also use a process called urea nitrogen salvage. This helps them keep their muscles strong. They do not lose much bone mass either. In 2016, Alina L. Evans studied brown bears in Norway. She found bears enter dens when snow arrives. Their heart rate variability drops as they enter hibernation.

Most people think only mammals hibernate. But other living things do it too. Some insects like the Polistes exclamans wasp show dormancy. Reptiles may undergo a winter dormancy called brumation.

Even some plants have a form of seed hibernation. Most birds do not hibernate, but the common poorwill does. This was first documented by Edmund Jaeger. For a long time, people thought swallows hibernated. This was written about by Gilbert White in 1789.

Hibernation is a wonderful way life survives the seasons.

Hibernation is a physiological state defined by minimal activity and metabolic reduction. This seasonal heterothermy involves a significant drop in body temperature. It also includes a much slower breathing and heart rate. Animals use this process to survive through winter months, a practice often called overwintering. Hibernation functions primarily to conserve energy when food sources are unavailable. To achieve these energy savings, an endothermic animal decreases its metabolic rate. This reduction in metabolism directly causes a drop in body temperature.

To prepare for this dormant period, animals must store massive amounts of energy. This period can last for days, weeks, or even several months. The exact duration depends on the species, the ambient temperature, and the animal's body condition. Larger species often become hyperphagic, which means they eat large amounts of food. They store this energy as fat deposits within their bodies. In contrast, many smaller species use food caching to store energy for later. Some mammals even hibernate while gestating young. For example, female black bears hibernate during winter to give birth to their offspring. These mothers increase their body mass significantly before entering their dens. This extra weight helps provide a warm environment for newborns. During hibernation, bears may lose between 15% and 27% of their pre-hibernation weight by burning stored fats.

Scientists categorize hibernators into two main groups: obligate and facultative. Obligate hibernators enter hibernation spontaneously and annually. They do this regardless of the ambient temperature or food access. This group includes many ground squirrels, rodents, and European hedgehogs. Their body temperature drops to near the ambient temperature of their surroundings. These animals experience periods of torpor interrupted by periodic, euthermic arousals. During these arousals, the heart rate and body temperature return to typical levels. Researchers are still studying why these arousals happen. One hypothesis suggests animals must warm up to pay off a "sleep debt." Another theory suggests these brief warm periods help restore energy or initiate immune responses.

Facultative hibernators behave quite differently from obligate species. They only enter hibernation when they face specific environmental stressors. These stressors include being cold-stressed, food-deprived, or both. A chipmunk is an example of a facultative hibernator. While it sleeps for long periods, its temperature does not drop to the low levels seen in obligate hibernators. It only truly hibernates if food becomes scarce. You can see the difference between these two types by looking at prairie dogs. The white-tailed prairie dog is an obligate hibernator. However, its close relative, the black-tailed prairie dog, is a facultative hibernator.

For many years, scientists debated whether bears were true hibernators. This was because bears only show a modest body temperature decline of about 5 degrees Celsius. This is much smaller than the large decreases seen in rodents. However, research in 2011 and 2016 refuted the idea that bears do not hibernate. Bears have unique ways of surviving long periods of inactivity. They can recycle their proteins and urine to stay hydrated and avoid muscle atrophy. They also use a complex process called urea nitrogen salvage, or UNS. In UNS, microbes in the gut break down urea into ammonia. The animal then uses this ammonia to synthesize essential amino acids like valine and leucine. This process also helps limit muscle wasting.

Bears also manage their bone health through hormonal changes. They increase CART hormone levels to inhibit bone-destroying cells. They also suppress sclerotin, a protein that stops bone formation, to prevent bone loss. Research by Alina L. Evans in 2016 provided new insights into brown bear behavior. She found that bears enter dens when snow arrives and temperatures drop below 0 degrees Celsius. Interestingly, their heart rate and temperature begin to drop several weeks before they enter the den. While the start of hibernation is driven by environmental cues, the arousal is driven by physiological cues. This means the bear's internal body signals help decide when it is time to wake up.

While hibernation is most famous in mammals, it appears in many other life forms. Ectothermic animals, such as reptiles and amphibians, cannot actively lower their own metabolic rates. Instead, they experience dormancy known as brumation. Some people use the term "brumate" specifically for reptiles. Many invertebrates also undergo metabolic suppression, which is called diapause. Insects like the Polistes exclamans wasp and certain beetles also exhibit dormancy. Even plants participate through a process called seed hibernation.

Most birds do not hibernate, though they may use short periods of torpor. For a long time, people believed swallows hibernated. This belief was documented by Gilbert White in 1789. However, the common poorwill is a known exception among birds. Its hibernation was first documented by Edmund Jaeger. Hibernation is not just a way to sleep; it is a complex biological system. It connects temperature, metabolism, and survival across the entire animal kingdom.