Bear Hibernation: Uncovering Black Bear Denning Secrets in Arkansas

Myron Means poked his head into the den and found the sow already awake. The crevice den was cramped, and she turned away from him as he raised the dart gun.

The black bear had chosen to den in a crevice on a south-facing slope, just off an overgrown logging road in the Ozark Mountains. She had been pregnant when she picked this spot in the fall and raked all the leaves toward the narrow entrance for insulation. By early March, her two cubs were six weeks old and growing fast.

Means is the large carnivore program coordinator for the Arkansas Fish and Game Commission, and he spends more than half the year in the field studying the state’s bear population. During winter, he conducts den checks with a team of wildlife professionals to examine sows and their cubs. Den checks help biologists like Means understand the reproductive and recruitment rates of their state’s bear populations. They also provide the opportunity to educate the public and dispel myths about bear behavior, including the question of whether bears hibernate.

After the anesthesia took effect, Means and his team got to work.

Where Do Bears Hibernate?

In his 27 years of studying black bears and their denning behavior, Means has found hibernating bears in various locations, such as rock crevices, briar patches, root balls, brush piles, burn piles, and tree hollows. Bears choose their dens based on their available habitat. Arkansas black bears in the Ozark Mountains, for instance, tend to den in rock crevices. Along the White River, where winter flooding is common, black bears make their dens high in the trees. In the Ouachita Mountains, bears choose dug-out dens under root balls. In the state’s southern Gulf Coastal Plain, they often choose briar thickets. Most people can walk right past a bear den without even knowing the animal is there.

“The bear tries to find as small and constricted a space as possible,” says Means. “That way there’s less energy for her to heat it and stay warm.”

Bears Are Not True Hibernators

When it comes to bear hibernation, most people imagine the children’s storybook version: Bears find a roomy cave in the fall, sleep all winter, and wake up only in spring. In reality, bears are not true hibernators. However, hibernation is the most common description for what bears do during winter. Hibernation is how bears have adapted to seasonal food shortages, low temperatures, and harsh winter weather. It’s also when they give birth.

Hibernation is characterized by three physiological changes: reduced metabolism, slower heart rate, and lowered body temperature. Bears, unlike groundhogs, do not experience a drastic drop in body temperature during hibernation. Groundhogs are one of the few mammals that truly hibernate, with their body temperature dropping below 40°F. Black bears, on the other hand, remain in a prolonged state of torpor or dormancy, during which their bodily functions slow down, but their body temperature only drops by a few degrees. Similarly, their heart rate significantly decreases during hibernation.

After darting the sow, Means and his team checked her vitals. The sow was breathing fewer than eight breaths per minute, and her body temperature was 97.8°F.

Bears sleep and are sluggish during the winter but don’t enter a coma-like state like groundhogs. They also move frequently in their dens. Biologists know this through den checks, den cameras, and collar-tracking programs. Bears in Alaska, for example, don’t sleep all winter long despite the longer den cycles and harsh conditions at higher latitudes. Staying awake during den cycles may be an evolutionary response to defense from predation, as male bears are known cub predators.

When Do Bears Hibernate?

The timing of bear hibernation depends on latitude. Bears living farther north enter their dens earlier in the fall and emerge later in the spring, with hibernation lasting five to seven months in Canada and Alaska. In contrast, bears living farther south have shorter denning periods. Bears in Arkansas, for example, are not greatly affected by inclement weather and don’t enter den cycles due to cold temperatures or snowfall. Food availability is the main factor that dictates bear hibernation.

“When a bear starts to go into an energy deficit, that’s when the mountain bears are going to start their denning cycles,” says Means. For bears in the Ozark and Ouachita Mountains, food availability plays a significant role in their denning behavior. In areas like south Arkansas where there is plenty of food, bears can delay denning until it’s time to give birth.

The timing of when bears emerge from their dens depends on their location.

“A lot of females that we had down [in south Arkansas] that had yearlings this year never really locked down on anyplace to den. Males are the same way. Males may not even have a den cycle in south Arkansas,” says Means.

While denning chronology is limited in Arkansas, Means performs den checks across the state to gain a better understanding of black bear denning behavior.

Understanding when bears typically den can help wildlife agencies set hunting season dates. The denning chronology of bears can vary greatly even within a few hundred miles. By studying denning behavior, biologists can determine the timing of hunting seasons based on the bear populations in different areas.

“I used to think that females go into the den sometime between early and late November based on food availability and their reproductive status. Then, in fall 2022, we had some females start a denning cycle in October. So that really raised the question: Are many of our bears already in a den cycle during bear season on an average year? We want to capture location information and timing going into den cycles,” says Means.

Den checks during bear hibernation provide valuable information about the reproductive and survival rates of bear populations, allowing biologists to adjust harvest goals.

“Bears [naturally] have very low reproductive rates. The drought [in Arkansas] substantially affected reproductive rates. Instead of seeing 80 to 90 percent of sows having full litters, we’re seeing about 35 percent. If there are drops in reproductive or survival rates due to environmental conditions occurring two or three consecutive years, we would adjust harvest strategies to mitigate that drop in population,” says Means.

Bear Hibernation Facts

While Means’ team monitored the sow’s vitals and took hair and blood samples, he examined the male and female cubs. Bear cubs, like human infants, have difficulty regulating their body temperature and are kept warm during den checks.

Hibernating Bears Don’t Eat

During a den cycle, adult bears typically don’t eat, drink, defecate, or urinate. These needs are suppressed due to the decline in bodily functions. Bears rely on their fat stores to survive during hibernation, which is why they need to eat abundantly in summer and fall. Nursing cubs, however, are an exception to this rule.

“While the females are in there with the [new] cubs, for about 4.5 months, they don’t eat, drink, defecate, or urinate. The only thing she’s eating is cub poop for three or four months,” says Means.

Sows Give Birth During Bear Hibernation

Bears have a two-year reproductive process, unlike other big game species that give birth every year. Peak breeding season for Arkansas sows usually occurs from May through early July.

“In the fall, at some point the sow’s body will trigger her to allow that [developing egg] to attach, and she will complete a gestation period. And that’s probably about the time she goes into a den cycle. She gives birth in mid- to late-January, nurses the cubs, and they emerge from the den in mid-April when they are big enough to follow. They spend all summer and fall learning how to be bears,” says Means.

When food becomes scarcer, the sow and her yearling cubs enter their den cycle together.

“Females with yearlings might come out and forage if there are a few acorns on the landscape. Yearling cycles are typically abbreviated den cycles, and the sow may come out in late February or early March. She’ll allow her yearling cubs to hang around with her for a while, but when the breeding season begins, she’ll start another two-year reproductive cycle,” says Means.

Pregnant sows have the longest den cycles, as they enter the den before other adult bears and emerge later. Once a sow stops having cubs, she’s nearing the end of her natural life. A healthy black bear sow in Arkansas will birth up to 16 cubs in her lifetime.

Hibernating Bears Lose Weight

During a denning cycle, adult bears can lose anywhere from 15 to 30 percent of their body weight. However, they remain healthy due to various evolutionary adaptations. Denning bears can recycle their metabolic waste by converting the nitrogen in their urea.

“They convert it to amino acids to feed their muscles. They can actually come out of a den cycle with more lean muscle mass than when they went in. It’s just the lifestyle they’ve evolved for, and they’re masters at it,” says Means.

Final Thoughts on Bear Hibernation

Bear hibernation behavior varies by latitude and habitat. Bears enter their den cycles based on available food and reproductive status. Adult bears don’t eat, drink, defecate, or urinate while in a denning cycle. Hibernating bears are sleepy and sluggish, but they frequently move in their dens and don’t sleep throughout the entire winter.