Hibernation: How Animals Power Down for Winter
It’s that time of year again—when animals retreat into dens, nests, and burrows to wait out the cold and darkness of winter. I always look forward to December 21st because the days finally start getting longer, bit by bit. But while many of us are craving more daylight, countless animals are preparing for the quiet, energy-saving state we call hibernation.
Most people know that bears hibernate, but what about foxes? Raccoons? Deer? Mice? Birds? Do all birds really fly south? And what actually happens inside an animal’s body during hibernation? Let’s explore.
What Exactly Is Hibernation?
According to a 2020 study by C. Frare, Cory T. Williams, and Kelly L. Drew titled Thermoregulation in hibernating mammals: The role of the thyroid hormone system, hibernation begins with a gradual decline in body temperature in early fall. As animals enter a state called torpor, they become physically and mentally less active.
One of the major players in this process?
The thyroid gland.
Thyroid hormones help regulate body temperature and metabolism in all mammals. During the transition to hibernation, these hormones shift to support extreme energy conservation.
In smaller mammals—such as mice and ground squirrels—hibernation involves periodic reductions in metabolic rate and body temperature from around October to March. They move between long periods of torpor and short periods of rewarming called interbout arousals, also known as euthermia.
Garst, Warren, 1922-2016, photographer
Why go through all this?
As Frare and colleagues explain, hibernation is an evolutionary strategy to conserve energy when food is scarce and temperatures drop.
Ectotherms vs. Endotherms: Who Can Hibernate?
In 2013, Fritz Geiser’s paper Hibernation described two broad physiological types of organisms:
Ectotherms
- Low metabolic rates
- Little or no insulation
- Body temperature depends on environmental temperature
- Includes most plants, reptiles, amphibians, and many insects
Ectotherms generally do NOT hibernate as mammals do. They often lack the internal heating ability to survive freezing temperatures… and many simply freeze to death.
Endotherms
- High metabolic rates
- Insulate themselves (fur, feathers, fat)
- Can maintain body temperatures between 32–42°C
- Includes most mammals, birds, some fishes, insects, and even flowering plants
Because maintaining body heat uses tons of energy—especially for smaller animals—many endotherms have evolved heterothermy, the ability to lower body temperature and metabolic rate, sometimes dramatically. This adaptation allows true hibernation.
During torpor, body temperature may drop to 0–20°C, heart rate and water loss decrease, and metabolism slows. Despite this, animals must still produce enough heat to prevent tissue damage.
“Dormice” by Kentish Plumber is licensed under CC BY-NC-ND 2.0.
Examples of Heterothermy in Nature
Here are some fascinating species that use torpor or hibernation:
- Mountain pygmy possum (Australian Alps)
Hibernates among snow-covered boulder fields for 5–6 months. - Tawny frogmouth
One of the largest birds known to enter torpor, especially at night or early morning. - Fat-tailed dunnart (Australia)
A small marsupial found in arid regions that regularly uses torpor to conserve energy.
These strategies allow animals to survive seasons when food would otherwise be too scarce to support their high metabolic needs.
Why Not Just Sleep? Or Migrate?
Being alive burns energy—walking, digesting, and even sleeping. Birds can migrate to warmer areas, but for many small mammals, migrating long distances would take far more energy than they could afford.
Evolution has carved out hibernation as the perfect winter survival strategy:
save energy, slow everything down, and wait for better times.
And believe it or not, hibernation isn’t just for winter.
Summer Hibernation: Estivation
Some animals hibernate in summer, especially in extremely hot or dry environments.
This is called estivation.
Examples:
- Certain species of tortoises
- Some amphibians and insects
- A few small mammals
These animals power down to avoid heat and dehydration instead of cold and hunger.
And some creatures enter daily torpor, a shorter, milder form of hibernation.
Bats are great examples—they can drop their body temperature for short periods to conserve energy.
Tortoise Hibernating Burrow
“Tortoise hibernation burrow” by U.S. Army Environmental Command is licensed under CC BY 2.0.
Do Zoo Animals Hibernate?
It depends on the zoo.
Some facilities intentionally trigger hibernation by adjusting temperature and lighting.
Others keep animals awake year-round by providing consistent warmth and food, which removes the environmental cues that normally trigger hibernation.
How Animals Prepare for Hibernation
Preparation is everything.
- Squirrels collect and store nuts.
- Bears pack on fat reserves.
- Some animals do both!
- Many species rely heavily on photoperiod—the changing length of daylight—to know when winter is approaching.
Animals must build up enough energy reserves to survive months without food.
What Happens Inside the Body During Hibernation?
A lot changes:
Breathing
- Drops by 50% or more
- Some reptiles temporarily stop breathing entirely
Waste
- Deep hibernators produce no feces
- They do produce urea, but they’re able to recycle it internally
- They prevent dehydration by extracting water from stored fat
The body becomes an energy-saving machine.
Hibernation: More Complex Than You Think
Hibernation isn’t just long sleep—it’s a dramatic, finely tuned physiological transformation. From thyroid hormones to torpor cycles, from possums in the Alps to birds in Australia, animals have evolved remarkable strategies to survive harsh seasons.
Next time winter settles in and the days grow short, remember that beneath the soil, inside hollow trees, and deep within snowy burrows, countless creatures are slowing their hearts, lowering their temperatures, and quietly waiting for spring.
https://www.sciencedirect.com/science/article/pii/S0960982213001310https://animals.howstuffworks.com/animal-facts/hibernation.htm