True hibernators survive winter by dropping their metabolism, heart rate, and body temperature far below normal and staying in that suspended state until warmer weather and food return. It’s one of the more extreme tricks in biology: an animal shuts its body down almost to the edge of death, then switches back on months later with no apparent harm.

What actually counts as hibernation

Scientists reserve the word “hibernation” for animals that meet three specific criteria — a sharply reduced metabolic rate, a slowed heart rate, and a lowered core body temperature sustained over an extended stretch — according to the National Park Service. That definition rules out a lot of animals people assume are hibernators. Black bears, for instance, enter a deep winter dormancy and can go months without eating, drinking, or eliminating waste, but their body temperature drops only a few degrees, so biologists classify their winter sleep as a lighter form of dormancy rather than true hibernation. Reptiles and amphibians go through a related but distinct process called brumation, which is triggered by cold rather than an internal seasonal clock, and during which they stop digesting food entirely.

The most extreme case: squirrels that supercool

The most dramatic hibernators are small rodents, and the arctic ground squirrel is the record holder. Its body temperature can fall from a normal 99 degrees Fahrenheit to as low as 27 degrees — several degrees below freezing — a state researchers call supercooling, in which the squirrel’s blood and tissue stay liquid without forming the ice crystals that would otherwise kill it. The National Park Service notes this is the lowest body temperature ever recorded in a mammal. Arctic ground squirrels spend seven to eight months of the year in this state, roughly from October through April in places like Denali National Park.

Why hibernators still wake up periodically

Even the deepest hibernators can’t stay that cold for the entire winter. Every two to three weeks, an arctic ground squirrel will shiver and shake for twelve to fifteen hours, generating enough metabolic heat to warm itself back to a normal body temperature of about 98 degrees before dropping back into torpor. Scientists still don’t fully understand why this periodic rewarming is necessary, though leading theories point to a need to restore immune function, clear metabolic waste, or get a burst of restorative sleep that simply can’t happen at such low body temperatures.

Hibernation’s shorter cousin: torpor

Not every energy-saving shutdown lasts all winter. Many small mammals and birds use torpor, a short-term version of the same strategy that can last just a few hours. Hummingbirds, for example, drop their body temperature and slow their racing heart rate overnight to survive cold nights without starving, then return to normal activity by morning. Torpor and hibernation sit on the same physiological spectrum — both trade activity for survival — and differ mainly in how long the animal stays down and how far its vital signs fall.

At its core, hibernation is an answer to a simple math problem: when food disappears and temperatures drop, staying active costs more energy than an animal can replace. Slowing the body’s engine to a near-idle lets hibernators survive on fat reserves built up over the previous summer and fall, sometimes losing a quarter or more of their body weight before spring. It’s a high-stakes strategy, but for arctic ground squirrels, bats, and other cold-climate specialists, it’s also one of evolution’s most effective ways to outlast a season with nothing to eat.