When the weather warms in spring, animals begin stirring from hibernation. But they haven’t spent the previous few months just catching up on beauty sleep. Hibernation is a complex physiological state that helps animals survive seasons when resources are low. Here are a few facts on this unusual adaptation, and the critters that have mastered it.
Hibernation’s Purpose Is To Conserve Energy
Hibernation is a form of torpor — a dormant state in which an animal’s body temperature cools and its heart rate and metabolism slow to conserve energy. Torpor can last as little as a few hours: Hummingbirds in the Andes, for example, cool their internal temperature up to 33 degrees Celsius and enter torpor overnight, saving energy until the following morning. Hibernation is basically torpor that lasts several weeks to several months. The degree of hibernation among animals can vary from the practically dead (the Arctic ground squirrel can lower its body temperature below the freezing point of water) to relatively active (bears don’t lower their body temperatures as much and periodically wake up during hibernation).
A Lack of Food Usually Sets Hibernation in Motion
Biologists used to think that cold weather was the signal for animals to start hibernating, since those in the temperate Northern Hemisphere disappeared into their dens when summer turned into fall and emerged when winter turned to spring. Then scientists discovered that numerous species in the tropics also hibernate, which suggested that hibernation was triggered by a seasonal lack of food instead of a change in temperature. These tropical species enter a hibernation-like state called estivation during hot and dry periods when water or food is scarce.
Hibernation Is Different From Sleep
When animals hibernate, they’re not just sleeping for weeks on end. “Light” hibernators like bears cycle between periods of rest, when their body temperature and functions are dormant, and brief periods of wakefulness when they change position, urinate, or even get some actual sleep. Female bears and other mammals may give birth and raise their young during this time. Deep hibernators, like some species of groundhog, mice, and bats, may remain practically motionless for months.
Hibernators Wake Up Hungry
Hibernating animals eventually wake up, signaled by the changing temperature of their environment or possibly by an internal “alarm clock.” The months spent in their cozy dens are actually not that relaxing: When animals emerge from torpor, they’re often underweight, tired, hungry, and thirsty. Their first post-hibernation acts are to drink water, hunt or forage for food, and size up potential mates.
A Huge Variety of Animals Hibernate
Numerous species of warm-blooded animals experience some degree of torpor, but only a small percentage of them are considered true hibernators. A single species of bird, the common poorwill, and a single fish, an Antarctic cod (which isn’t warm-blooded, but does produce antifreeze-like proteins in its body), are known to hibernate. The practice is much more common among mammals; hibernating mammals include echidnas, insect-eating bats, at least one species of armadillo, the fat-tailed dwarf lemur, badgers, ground squirrels, marmots, jumping mice, dormice, and black and brown bears.
There Have Been a Few Cases of Human “Hibernation”
You may have noticed one mammal that doesn’t hibernate — us. But there are a handful of cases in which humans have endured a lethally low body temperature and lived, with no lasting effects. The most famous is the ordeal of Mitsutaka Uchikoshi, a 35-year-old Japanese civil servant, who slipped on a mountain trail and broke his hip in October 2006. He was rescued after 24 days suffering from extreme hypothermia “similar to hibernation,” his doctors said. After nearly two months in the hospital, he emerged with no residual injury. In 2012, a Swedish man was stranded in his snowed-in car for two months but survived, despite having severe hypothermia and no food, possibly due to having entered a torpor-like state.
Hibernation Might Help Humans Get to Mars
Research into animal hibernation has the potential to help humans. Understanding why hibernators can withstand extremely low body temperatures and slowed metabolism without injury might give us clues for recovering from heart attacks, preserving human organs for transplant, or conducting complex surgeries. Scientists are even experimenting with “induced hibernation” as a way to conserve astronauts’ energy on long journeys through space, and to reduce the amount of resources needed on future missions to Mars.