In the coming weeks, bears, squirrels, marmots, and many other beasts will emerge from hibernation. An extended period of inactivity might sound relaxing, but hibernation is very different from rest.
Actually, it would be more like having a stroke. Oxygen flow to the gray matter of hibernators can drop to as little as 2 percent of normal.
Metabolic activity and heart rate also plummet. A ground squirrel’s heart rate can drop from 300 beats per minute to three or four ticks in the same span during hibernation.
It’s not just for winter: In tropical Madagascar, the fat-tailed dwarf lemur crawls into tree-holes and shuts down for an average of seven months to deal with a dearth of food and water during the dry season.
Warm-weather hibernation, or estivation, is also used by lizards, snails, and turtles.
Even fish can hibernate. Notothenia coriiceps, informally known as Antarctic cod, cuts its metabolism by two-thirds and burrows under the seabed for days at a time during dark Antarctic winters.
A state of suppressed physiological activity is known as torpor, and a handful of mice, bats, and birds employ it on a daily basis. The ruby-throated hummingbird can reduce its metabolism tenfold during the night, then pop up and fly in the morning.
Hibernation is basically an extended form of torpor, but torpor alone does not qualify as hibernation. Got it?
Animals in torpor can drop their body temperatures to startling extremes. The Arctic ground squirrel cools down to 27 degrees Fahrenheit.
The squirrel avoids turning into a rodent popsicle by flushing ice nucleators—proteins that normally facilitate freezing—from its blood.
Mammals that go into true hibernation have to find ways to warm up every few days or weeks.
The Arctic ground squirrel does it by tapping thick stores of brown adipose tissue, a kind of fat and muscle common in human infants. Metabolic reactions within the tissues release energy, warming the heart, lungs, and brain by a few degrees. Then the animal shivers to lift its temperature back to a normal 98 degrees, but just for half a day.
Marmots wake from hibernation only to lapse into quick (12- to 15-hour) naps that help them get the juices flowing again. During these periods of rest, brain activity resembles that of normal slumber. Then the marmots drop back into near brain-dead hibernation for three weeks.
What to expect when you’ve checked out for the winter: A pregnant American black bear can give birth without ever emerging from hibernation.
She doesn’t even need to rouse herself to care for her young, instead nursing her cubs for months by drawing on her reserves of stored fat. Call it the ultimate postpartum weight loss plan.
But she remains protective. A hibernating mama bear’s heart rate can spike and wake her if you come within about 50 feet of her den, according to engineer Timothy Laske, who has outfitted the animals with radio-transmitting collars to monitor their vital signs. “You cannot sneak up on a hibernating bear,” he warns.
Stumbling upon a hibernating bear in the woods is more likely than you might think. Some winter openly in five-foot-wide nests made of branches and foliage.
When spring comes, or droughts end, most hibernators emerge from their dens or nests without significant muscle atrophy or bone density loss.
How do they do it? Seth Donahue, a biomedical engineer at Colorado State University, suspects that the parathyroid hormone, which regulates calcium uptake, helps maintain bone density. He is currently testing the hormone on human bone cells in his lab.
Hibernation is probably not in the cards for people. But Donahue’s research could lead to new treatments for spinal fractures and osteoporosis.