Over the winter, a few fuzzy faces will be absent at the Minnesota Zoo. That’s because brown bears and American black bears will be sleeping behind the scenes as they experience the natural, seasonal state of torpor.

Torpor is a little different than hibernation. Bears don’t experience as deep a state of sleepiness as small mammals and amphibians that hibernate. Animals that hibernate in the wild, like groundhogs and wood frogs, enter a deep and constant sleep to save energy and survive the winter.

Like their wild relatives, the brown bears at the Minnesota Zoo will spend the winter in a state of sleepiness and inactivity called torpor. Torpor is a natural behavior that helps northern bear species, like brown bears and black bears, save energy and survive when food is scarce in winter.

When bears are in torpor, their heartbeat, breathing, body temperature and metabolism slow down. They need fewer calories and sleep more. But they’re resting more lightly than hibernating animals, so even in their drowsiness, they can still get up to eat a snack or eliminate waste.

As temperatures and daylight decrease in autumn, bears’ bodies signal them to eat more food and put on weight. When winter settles in, so do bears in their dens. This behavioral adaptation helps northern bear species survive when food is scarce.  Bears of the southern hemisphere, like the sun bear, never developed this adaptation because food is available year-round in warm climates.

Bear Healthcare

It takes a large team to care for bears, including veterinarians, veterinary staff, an animal nutritionist, and Zookeepers on the Minnesota Trail and Russia’s Grizzly Coast. All contribute to health maintenance and the fostering of bears’ natural behaviors, like torpor.

Dr. Anne Rivas, DVM, DACZM, serves as Director of Animal Health. Since she joined the Minnesota Zoo in 2021, she’s been hard at work conducting physicals on animals to get a baseline for their condition. During exams, Dr. Rivas and the team use imaging to view the animals’ internal organs, draw fluid for blood tests and urinalysis, give vaccinations, and check and clean teeth.

The bears’ health checkup in 2022 detected signs of aging, especially in the 14–16-year-old brown bears who show some symptoms of arthritis. In support of the bears’ health, the team determined it was the right time to encourage the bears to experience torpor instead of staying active all winter.

“Our ongoing goal is to provide for their long-term health,” Dr. Rivas says. “By encouraging the bears to go into torpor, there will be many health benefits as they go through the natural cycle of eating in the fall and being sleepy in the winter.”

This is a new practice for the Minnesota Zoo, but both Dr. Rivas and Kelly Kappen, Animal Nutritionist, have experience supporting bears in torpor at other zoos where they’ve worked.

Kappen, who joined the Minnesota Zoo in 2022, says torpor is complicated to manage, and takes thoughtful adjustments of the bear’s diets and environment to discourage them from staying up as they have in past winters.

Both Rivas and Kappen cite a big benefit of torpor: as they sleep, the bears will gradually lose the cold-weather weight they gained in the fall. This will be gentler and easier on the bears’ joints and bodies.

“A big concern with bears in zoos is weight gain because they don’t have to seek out food like in the wild,” Kappen says. “They’re omnivores and they can and do eat everything! It’s important to carefully monitor their weight.”

The American black bears on the Medtronic Minnesota Trail will eat more food and put on weight in the fall. In wintertime, they will remain in their dens in a resting state of torpor, during which they will gradually, gently lose weight.

Occasionally, keepers can register weight when the bears voluntarily step on a scale. But mostly the team uses body condition scoring, in which they visually assess the bears for where they might have extra fat deposits in key areas of their bodies, like the shoulders and hips. Then they assign them a score on a scale of 1–9, emaciated to obese.

“Our bears are 6–7 going into torpor, and we aim for a score of 4–5 coming out,” Kappen says. “This is the normal, seasonal cycle for wild bears.”

In addition to using body condition, Zookeepers monitor how much food the bears eat, noting if they eat every last bite or leave crumbs behind. All the data collected—weight, body score, food intake, other observations of behavior—is closely tracked so that Zoo staff can support the bears’ health and wellbeing during torpor and throughout the year.

Snoozing Behind the Scenes

During torpor, Zookeepers provide bears ample straw bedding and reduce the lights, temperature, and noise levels in their “bedroom areas,” Dr. Rivas says. Zookeepers check on the bears daily—without disturbing their slumber.

The bears cozy up with sleeping companions, too.

“Our bears, because of their close social nature, will den together,” Dr. Rivas says. “While this wouldn’t be the case for adult bears in the wild, it’s normal for mothers to give birth during torpor and tend to and nurse cubs in the den.”

Kappen reports that the bears will get up most days to drink and stretch. Snacks such as peanuts and romaine lettuce are provided. They still defecate, but only about every other day during torpor, instead of two times daily in spring, summer, and fall.

“They always have some food available if they get up to nibble,” Kappen says. “That would be true for bears in the wild, too. They would get up, browse for food, and then go back to the den. If food was scarce, they would just go back to sleep.”

Let Sleeping Bears Lie

As winter thaws and the bears awake, Kappen will be ready with a spring diet that’s akin to that of wild bears. Keepers will offer an increasing amount and variety of lower-calorie foods like leafy greens. The black bears especially like cucumbers and tomatoes, she says. Foods like these, with high moisture and fiber, help the bears feel full and satiated.

The brown bears will remain behind the scenes on Russia’s Grizzly Coast until warming temperatures and increased daylight in late winter and early spring signal them to become more awake and active.

“This mimics springtime in the wild, when the berries and forage don’t all ripen at the same time,” Kappen says. “It’s a gradual onset of food sources and most are plants.”

Taking their cues from warming temperatures and increased daylight, the bears will begin to stir. But unlike humans who must move from fast asleep to 60-mph commutes, bears can take their time going from the grogginess of torpor to fully up and active.

“How long torpor lasts will depend on the environment and weather, but it might be different for each individual bear, too,” Dr. Rivas says.

There are no alarm clocks in the bears’ dens. “We won’t rouse them,” she says. “They’ll wake up when they’re ready.”

Minnesota seasons don’t start and stop according to a calendar, and neither do bears in torpor.