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Which animal can live without food for 2 years?

Many animals have adapted the ability to go for extended periods without food. However, one animal stands out as being able to survive without eating for up to 2 years – the bear. Hibernating bears can go without food for up to 7 months while they sleep through the winter. During this time they live off their fat stores and maintain muscle strength even while inactive. Let’s explore how bears can pull off this incredible feat of survival.

How Do Bears Hibernate Without Food?

Bears are able to hibernate through the winter without food by undergoing some amazing physiological changes. Here’s what happens in a bear’s body to allow it to hibernate without food for so long:

  • Bears build up a thick layer of fat during the summer and fall. This provides them with energy reserves that can be slowly metabolized while they hibernate.
  • Their heart rate drops from around 55 beats per minute to just 9 beats per minute. This lets them conserve energy.
  • Their body temperature decreases slightly from about 37°C to around 33°C. This reduces their metabolic rate so less energy is used.
  • They do not defecate or urinate during hibernation. Their body is able to recycle waste to extract any remaining nutrients and water.
  • Although they are inactive during hibernation, bears maintain their muscle strength and mass. Special proteins prevent their muscles from atrophying.
  • Pregnant female bears give birth during hibernation and nurse their cubs. This is fueled by their fat stores.

These incredible adaptations let bears conserve energy and survive for months without eating a single bite of food. They emerge from their dens in spring ready to forage and rebuild their fat stores again.

How Long Can a Bear Go Without Food?

The length of time bears can hibernate without food depends on the species:

  • Brown bears – hibernate for 5-7 months each winter.
  • Black bears – hibernate for 2-6 months in winter.
  • Polar bears – hibernate for 4-5 months in winter dens.
  • Sun bears and sloth bears – do not hibernate but may rest more in winter.

Of these, the brown bear wins the prize for the longest hibernation without food. Brown bears, also known as grizzly bears, enter their dens in October or November and emerge in March or April. During this 5-7 month hibernation they live entirely off their fat stores and do not eat or drink.

How Do Hibernating Bears Avoid Muscle and Bone Loss?

One of the biggest threats of being immobile and inactive for so long would normally be devastating muscle atrophy and bone loss. However, hibernating bears have special adaptations that prevent this.

Hibernating bears continually recycle urea into proteins that maintain their muscle strength. They also avoid bone loss by conserving calcium and maintaining bone remodeling. Researchers found that bears do not lose bone or muscle mass despite being immobile for nearly half of the year without food.

Understanding these protective mechanisms could provide insights that help prevent muscle wasting and osteoporosis in humans. Hibernating bears provide a unique natural model for maintaining strength and health during long periods of inactivity and fasting.

Comparison to Other Hibernating Animals

Bears have some of the longest hibernation periods of any animal. Here’s how other hibernating mammals compare:

Animal Hibernation Length
Bears 5-7 months
Ground squirrels 5-8 months
Hedgehogs 3-4 months
Hamsters 4-6 months
Bats Up to 6 months

As you can see, bears hibernate longer than any other mammal. Only certain frog and turtle species spend more time inactive and without food.

In addition to longer hibernation, bear adaptations like recycling urea into useful proteins are unique. Most small hibernating mammals survive by entering a state of torpor where their body temperature drops close to freezing.

Do Hibernating Bears Wake to Feed?

Unlike some smaller hibernating mammals, bears do not rouse themselves to feed during the winter months. Bears remain in deep hibernation continuously for 5-7 months, living solely off their fat reserves. Their heart rate, body temperature and metabolic rate drop very low.

Some smaller mammals like ground squirrels and hamsters will periodically come out of torpor during hibernation. These brief arousals let them feed to maintain their fat stores. But food is not usually available to bears in the dead of winter. Their profound hibernation adaptations allow them to remain inactive the entire time.

Surviving Without Water

Surprisingly, hibernating bears can go without drinking water for those 5-7 months of winter hibernation. The reason they can survive without water for so long comes down to some incredible kidney adaptations.

During hibernation, a bear’s kidneys are able to utilize metabolic water produced by breaking down their fat tissue. By recycling water, bears maintain adequate hydration levels in their blood, tissues and cells throughout hibernation with no additional water intake.

Do Hibernating Bears Lose Weight?

Yes, hibernating bears do lose significant weight over the winter due to metabolizing their fat stores. They may lose 15-30% of their body weight before emerging in spring.

For example, a 400lb bear may lose around 100lbs over a 5 month hibernation. This is why bears are so motivated to eat as much as possible and build up fat in late summer and fall before their long winter fast begins.

How Do Bears Fatten Up Before Hibernation?

In late summer and fall, bears spend up to 20 hours a day eating to gain the fat they need to survive winter. A bear’s diet during this time consists of:

  • Nuts
  • Berries
  • Fish
  • Insects
  • Roots
  • Seeds
  • Small mammals
  • Carrion

This high-fat and protein-rich diet lets them rapidly pack on body fat. Bears may eat up to 40,000 calories a day in autumn to fatten up. They need to double their body weight in some cases to provide sufficient fat reserves.

Why Don’t Hibernating Bears Need Food?

Hibernating bears get away with not eating for 5-7 months for a few key reasons:

  1. Large fat reserves provide energy to metabolize.
  2. Lower heart rate and body temperature conserve energy.
  3. Kidneys recycle water so they don’t get dehydrated.
  4. Muscles and bones are preserved from atrophy.
  5. Their digestive system shuts down and doesn’t require feeding.

Together, these amazing bear adaptations let them tap their fat supply and internally recycle nutrients and water to meet their needs. No external food or water is required!

Do Mother Bears Nurse During Hibernation?

Female bears give birth during winter hibernation and nurse their cubs while still fasting. The female’s body relies on stored fat to produce milk for her cubs throughout the long winter.

The cubs will nurse for 2-3 months while in the den. After emerging in spring, they continue to nurse on the mother’s milk until they are 6-8 months old. The high-fat milk provides the crucial nourishment the cubs need to grow and keep warm.

How Do Hibernating Bears Stay Warm?

Despite being confined in dens in freezing winter conditions, hibernating bears manage to stay warm thanks to thick insulating fur and fat. Breaking down their fat reserves actually produces heat as a byproduct, which also helps maintain their body temperature.

Hibernating mother bears will pull their cubs in close to keep them warm. Cubs don’t have enough body fat to produce sufficient heat yet. The mother’s fat and fur insulates the cubs from the cold.

Do Hibernating Bears Ever Wake Up?

Hibernating bears remain in continuous deep hibernation all winter long without waking. However, pregnant female bears will give birth during this hibernation period. The birthing process interrupts their hibernation temporarily.

Apart from brief birth interruptions, bears stay asleep from fall until spring. Changing weather cues in spring signal them to emerge from their dens hungry and ready to forage in the open again.

How Do Bears Know When to Wake Up?

Bears rely on various signals to know when to conclude their hibernation in springtime. Rising temperatures prompt changes underground in the bear’s den that eventually rouse them.

For example, melting snow may cause flooding in the den. Declining fat stores cause bears’ body temperature to increase. Cubs become more active and may start making noise.

These clues alert the bears that winter is ending and food will soon be available. Their amazing internal clock brings them out of hibernation once the seasons begin to change.

Effects of Hibernation on Bears

Hibernating without food or water for 5-7 months amazingly has few ill effects on bears. Some of the effects include:

  • Weight loss of 15-30% of body weight
  • Mild muscle atrophy in spite of adaptations
  • Temporary changes in blood chemistry and composition
  • Stiffness in joints until they become active again
  • Lethargy and increased sleep for a few weeks after emerging

Overall though, bears exit hibernation strong and healthy. Their kidneys quickly return to normal function. After a bit of sleepiness and joint stiffness from disuse, bears revert to typical health and activity when ample food is available.

Why Don’t Humans Hibernate?

Humans do not naturally hibernate for several reasons:

  1. We lack the fat stores and adaptations bears have to survive months without food.
  2. Our bodies would suffer from profound muscle atrophy without exercise for that long.
  3. We are not adapted to reuse water and nutrients like bears do.
  4. Our digestive system requires more frequent nourishment.
  5. Reduced waking hours would limit our productivity and survival.

While bears can hibernate because it helps them survive periods of food scarcity, continuous activity and wakefulness provides more advantages for human survival. Our bodies work best with regular nutrition and activity.

Could Humans Hibernate with Help?

While we do not naturally hibernate, some scientists think humans could possibly hibernate with advanced assistance someday. Potential methods include:

  • Using drugs to induce long-term sleep and reduce metabolism
  • Applying mild electric shocks to prevent muscle atrophy
  • Providing amino acid solutions via IV to maintain nutrition
  • Using cooling blankets to reduce body temperature
  • Installing catheters and collection bags to manage waste

With extreme interventions like these, hibernation-like states may be possible. However, experts note this raises major medical ethics concerns and the risks likely outweigh any benefits for most people.


Of all animals on Earth, bears reign supreme in their ability to hibernate without food or water for up to 7 months. Their incredible physiological adaptations allow them to live off their massive fat stores through long winters of deep sleep.

Understanding how bears can hibernate without eating or drinking for so long provides fascinating biological insights. While humans are not equipped to hibernate naturally, bears will continue showcasing their amazing survival abilities each winter.