Sleeping Beauties

Bears are perfectly evolved to nap all winter long.

Sleeping Beauties

Bears are perfectly evolved to nap all winter long.    

By Kylie Mohr

Photo by Drew Rush

PAWS CRUNCHING THROUGH the foliage, the female grizzly is on a mission. She lumbers through the forest, sniffing out different food sources in her annual quest to prepare for winter’s hibernation. Her nose, equipped with a sense of smell estimated to be about seven times greater than a bloodhound’s, can sense food miles away. And with a territory of 50 to 300 square miles, she needs that powerful nose. Carrying a blastocyst or two (see sidebar), if she bulks up enough these will mature into cub embryos she’ll give birth to while in her den this winter. To do this, Ms. Grizzly might prey on elk calves when they’re still small, or, in areas around Yellowstone Lake, feed on spawning cutthroat trout. Sometimes she’ll cache her food for up to several days at a time or plunder other animal’s caches—like squirrels’ middens of whitebark pine  nuts. She’s not picky. As the summer progresses, she’ll eat everything from succulent grasses to dandelions, thistles, ants, globe huckleberry, and false truffles. Maybe she’ll even scavenge wolf-killed ungulate carcasses. 

Every winter, bears combat the season’s long, dark days and shortage of food by doing what some of us dream of: hibernating. Bears, including celebrity Grizzly 399, a 22-year-old sow known for raising her cubs around the willow flats near Jackson Lake Junction in Grand Teton National Park (GTNP), prepare for hibernation by eating as many calories as possible. While the most important bear food in September and October is whitebark pine nuts, they’ll also eat fall foods like pondweed root, sweet cicely root, sedges, bistort, yampa, strawberry, grouse whortleberry, buffaloberry, clover, horsetail, ungulates—including carcasses—and army cutworm moths.

The average bear loses 15 to 30 percent of its body weight over the five(ish) months it hibernates. That’s anywhere between 30 and 120 pounds for the average sow and between 30 and 210 pounds for the average male. 

In the months leading up to hibernation, “They basically just eat nonstop,” says Kate Wilmot, Grand Teton National Park bear management specialist. Wilmot isn’t exaggerating: “They’ll eat for 20 to 24 hours a day.” The amount of food consumed varies based on the type and quality of food, as well as the age, sex, and mass of a bear; bears in the Greater Yellowstone Ecosystem can consume up to 20,000 calories a day if they find high-quality foods like army cutworm moths.

Photo by Ryan Dorgan

EATING THIS MUCH has a name: hyperfasia. “Hyper” means extreme activity or highly active; the suffix “phagia” (or “fasia”) has to do with eating. Bears begin to enter hyperfasia, a state in which they exhibit a continuous need to eat, around mid-July. By September, eating can consume—no pun intended—about 90 percent of their day. “Their primary goal in life at that point is to just put on fat,” says Kerry Gunther, Yellowstone National Park bear management specialist. “They eat as much as they can. They can gain several pounds a day.” 

While spending all of one’s time searching for food and eating is indeed onerous, hyperfasia prepares bears for their upcoming hibernation. Contrary to popular belief, bears do not hibernate as protection against cold weather, but because of the lack of food. “The further north you go, the longer bears hibernate,” Gunther says. “A lot of people think it’s the cold weather, but really, bears put on a thick layer of fat and have thick fur coats. They’re pretty well adapted to cold weather. It’s the lack of food.” Wilmot adds, “There’s nothing for them to eat.”

Bears in Mexico or Florida might den for a couple weeks, or not at all, while bears closer to the Arctic Circle in Alaska might den for half a year. Bears in Jackson Hole usually begin readying their dens between October and January; this is the time when their usual food sources become increasingly difficult to find (more and more snow buries them). But, “Bears are all individuals,” Gunther says. “Some bears build dens weeks in advance and others seem to do it at the last moment.” Bears differ in when they enter their dens, too: “Some are out until Christmas,” Gunther says. “Those [bears] are usually cleaning up wolf-killed ungulates.” In the 2011-12 winter, one sow grizzly didn’t den up with her yearling cubs until January. (While most bears are solitary in their dens, a female will often reden with her cubs as yearlings and sometimes even as two-year-olds before the kids go off on their own.) 

THE EXTRA WEIGHT bears put on during hyperfasia gives them a better chance of getting through winter’s food shortage. Increased weight gives them a lower surface area-to-mass ratio and cuts their metabolic rate to about 50 to 60 percent of normal. This means that, despite the frigid temperatures, they lose body heat more slowly. Still, they are using energy. “They’re living off body fat,” Gunther says. Overwinter mortality is rare for adults, bear management specialists say. It’s higher for younger animals, particularly the young of that year. 

As impressive as bears’ ability to live off fat for months on end is, more unique is that they’re able to build protein while doing it. “If a human was bedridden for five months, [her] muscles would all atrophy,” Gunther says. Bears break down the urea produced from the fat metabolism, and the resulting nitrogen is used to build protein. “A bear recycles waste products into protein,” Gunther says. This ability allows bears to maintain their muscle mass through hibernation, something that’s caught the eye of a team of European biomedical researchers, biologists, and neuroscientists who study how bears hibernate. They’re looking at bears with an eye toward human-related benefits, such as how to maintain muscle mass and bone density for space travel
to Mars. 

Nap time doesn’t last forever, but when bears emerge from their dens varies tremendously. Deep snow and male bears present obstacles for newborn cubs. They have to be big enough and strong enough to be mobile. In 2017, Grizzly 399 had twin newborns, and the three weren’t spotted (by a road in GTNP) until mid-May. Bigger males are usually the first bears to wake up. This happens here as early as mid-February. Females with yearlings also might come out around this time. Wilmot says that “by mid-March, about 50 percent of males are awake in this ecosystem. The last to wake up are the females with the small cubs.” In 2017, Grand Teton National Park biologists saw a sow grizzly with two two-year-olds on March 29. Sometimes bears emerge and then decide to hit snooze. Wilmot says, “It’s not abnormal for a bear to take a cruise or a walkabout and decide they’re going back to bed.” 

The first bear sighting of 2018 in Yellowstone National Park, was an 11-year-old male grizzly. He was spotted March 6. Whenever a bear wakes up, there’s no doubt it’ll be hungry. And then the cycle, in tune with nature’s clock, begins again.

Hibernation Timeline


Both black and grizzly bears breed between mid-May and mid-July—before hyperfasia kicks in—and are truly pregnant for only about two months. Bear cubs are usually born in January, while moms are hibernating and in a semiconscious state. If this math—mating in the early summer, a two-month gestation period, and birth in January—seems off, you’re right. Bears have evolved a unique reproductive trait called “delayed implantation.” After mating, the fertilized egg develops into a tiny ball of cells—a blastocyst—and then it just hangs out, unattached and dormant, in the bear mom’s uterus. If a sow gains enough weight during hyperfasia—150 pounds is usually enough—around late November the blastocyst will implant in her uterine wall and continue to grow. (If the female fails to gain enough weight, the blastocyst will not implant, and pregnancy is terminated.)

When born, bear cubs are small, blind, and almost hairless (if not completely so). A black bear baby weighs about as much as a stick of butter. Cubs immediately find their moms’ breasts and begin to suckle. “A bear’s milk is really high in fat, so those [cubs] will just grow, grow, grow, grow,” Wilmot says. 

Photo by Ryan Dorgan


A long time ago, some wildlife biologists did not consider bears to be “true” hibernators. That’s since changed, Wilmot says, and “there has not been a debate for some time.” Gunther agrees. “Bears really are true hibernators.” Animals that have always been recognized as true hibernators include chipmunks and ground squirrels. “[Bears] do all the same things” as these species, Gunther says.

Gunther says a better distinction is shallow versus deep hibernators. A deep hibernating animal has a body temperature that dips below 40 degrees Fahrenheit, and Wilmot says, “Their bodies really take a dive in terms of biological parameters,” which makes it very difficult (and slow) for them to awaken. But a shallow hibernator, like a bear, can be easily aroused. A hibernating bear’s metabolic rate is about 30 percent of normal, and heart rate is about 20 percent of normal. While a deep hibernator’s metabolic rate depends on species, body mass, and ambient temperatures, small rodents, like ground squirrels, can reduce their heart rates from more than one hundred beats per minute to less than five beats per minute. 

While the possibility of a bear waking up to deal with an emergency, like a predator at the door, is a rare occurrence, the potential is always there. Their relatively high hibernating vitals mean they’re prepared and can respond if need be. “They can wake up quickly say, if wolves are trying to dig into the den to eat the cubs,” Gunther says.