It’s finally spring (despite the fact that as I write this there’s as much snow on the ground as there has been all winter) and that means that many lucky creatures are just now waking up from their long winter sleep.
Deep in burrows or caves, various ground squirrels, marmots, woodchucks, shrews, hedgehogs and bats lie mostly dormant all winter long, not emerging until food and heat are both once more plentiful.
During hibernation, as this state is called, an animal’s metabolic rate may slow to as little as one percent of its active level and its heart rate and breathing drop, decreasing the need for food and water to a minimum, enabling the animal to survive for long periods on fat stored in its body. (Of which there can be quite a lot: squirrels, for instance, eat so much before hibernating that their body weight may double.)
It takes between 12 hours and a full day for an animal to enter hibernation, during which time its body temperature falls. One of the unique things about hibernation is that while it’s hibernating, an animal that’s normally warm-blooded — that is, it maintains a constant body temperature no matter what the outside temperature is — becomes essentially cold-blooded, with a body temperature that fluctuates with the outside temperature. In fact, a hibernating animal’s body temperature, normally comparable to our own toasty 37 degrees Celsius, may drop to just above freezing.
By that point, the hibernating animal is in a deep torpor — a lot deeper than ordinary sleep, which is interspersed with periods of great brain activity. By contrast, the brains of hibernating animals show no detectable activity at all.
Obviously the brain doesn’t really shut down completely, though, because many things can wake a hibernating animal, including noises, lights, odours and severe cold spells (if its body temperature threatens to drop even lower than freezing, an animal’s temperature-control mechanisms kick back in and start warming things up). The hibernating brain is apparently sort of like an empty office building with an automatic alarm system: if something goes wrong, it calls staff back from their winter holiday.
Even in the absence of alarms, many hibernating animals wake up periodically o eat food they gathered and stored in the autumn., then sink back down into their torpor. Nobody is sure what triggers hibernation. Decreasing temperatures, food shortage, shorter days and hormonal activity all seem to play a part. It’s thought the state is induced by a blood substance called hibernation induction trigger (HIT).
HIT hasn’t been completely isolated or analyzed, but some of its functions have been. We know that HIT acts much like an opiate to lower the metabolic rate. It also acts as a cell protectant, and among the most important cells it protects are those of the heart. Scientists continue to study HIT and related chemicals in detail because in the laboratory HIT has been shown to extend the shelf life of organs awaiting transplant. It could also someday point the way to a new emergency treatment for heart attacks, by protecting the heart from damage caused by the interruption of its blood supply. Andit might even be able to trigger hibernation in non-hibernating animals such as humans, which could be useful in certain medical and surgical procedures or on long space flights.
Some animals sort of hibernate every day. Hummingbirds and several species of swifts, poorwills, and bats, which have high metabolic rates but extremely small bodies, spend a portion of each day in a state of torpor rather than normal sleep, as a means of conserving energy–otherwise, a hummingbird could actually starve to death overnight.
Desert animals display another kind of dormancy, called estivation. It occurs during dry seasons, when food and water become scarce. Body activity and metabolic rate decrease, but body temperature doesn’t drop much because deserts don’t get very cold.
Some animals that we think of as hibernators don’t really hibernate in the truest sense of the word. Bears don’t hibernate the same way small mammals do. They do, however, enter a prolonged winter sleep, during which their metabolic rate decreases. It’s not the torpor of true hibernators, however; they wake up periodically and frequently cubs are even born during this period.
Other animals that exhibit this kind of winter sleep include skunks, pocket gophers and chipmunks.
People, however, can’t sleep the winter away. Or so scientists say.
Personally, I’d like to give it a try.