Do Bears Hibernate? What Really Happens in a Winter Den

What happens to a bear’s body in the den?

A hibernating bear slows its whole body down. In black bears, the heart rate falls from about 40–50 beats per minute in summer to as low as about 8–20, sometimes with long pauses between beats. Breathing slows to just a few breaths a minute. A 2011 study of black bears denning in outdoor enclosures in Alaska found that metabolism dropped to about 25% of the normal resting rate.

The surprise was body temperature. Small hibernators such as ground squirrels cool to just above freezing, but a bear’s core temperature falls only from about 37–38 °C (99–100 °F) to around 30–35 °C (86–95 °F). Because a bear is large and well insulated, it can cut its energy use dramatically without getting cold.

Even more remarkable, a denning bear does not urinate or defecate for months. It recycles urea, a waste product, back into proteins, which protects its muscles. Its bones do not weaken the way a bed-bound person’s would, and it loses far less muscle strength than an inactive human. Waste in the gut forms a firm fecal plug that is passed in spring.

Is it true hibernation or torpor?

For decades, many biologists said bears were not “true” hibernators because their temperature stays high and they wake easily. Groundhogs and ground squirrels, by contrast, drop their temperature by more than 30 °C (54 °F) and wake every week or two. You can compare them in do groundhogs hibernate?

Today hibernation is usually defined by the deep, controlled drop in metabolism rather than by temperature alone. By that measure bears qualify, and their strategy is extraordinarily efficient: the metabolic slowdown even continues for weeks after they leave the den. Some writers still use “torpor” or “winter sleep” for bears, so you will see all three terms.

How long do bears stay in their dens?

Den time depends mostly on climate and food. In Alaska and northern Canada, black and brown bears may stay in their dens for six to seven months, sometimes longer. In the Appalachians or the Pacific Northwest, three to five months is more typical. In Florida and Mexico, many black bears, especially males, stay active through most of the winter, while pregnant females still den to give birth.

Bears usually enter dens in October or November, when food runs out and snow arrives. A study of brown bears in Sweden found that bears went into their dens around the time snow fell and temperatures dropped to about 0 °C (32 °F), although their heart rate and activity had begun to fall weeks earlier. Adult males tend to come out first in spring, and mothers with newborn cubs last.

How do bears prepare for winter?

The key is fat. In late summer and autumn, bears enter a phase called hyperphagia, which means “excessive eating.” A black bear may feed for most of the day and night on acorns, nuts and berries, and a brown bear can gain a great deal of weight from salmon and berries. By the time they enter their dens, many bears are 30% heavier than they were in spring.

That stored fat fuels the whole winter, and it also provides the water bears need, because breaking down fat produces water. Over the winter, a denning bear commonly loses around 15–30% of its body weight. If a female has not stored enough fat, her body may not allow a pregnancy to continue.

Cubs born in the den

Bears give birth in the middle of winter, while the mother is in her den. Black and brown bear cubs are born in January or February, and polar bear cubs in November to January. The cubs are tiny and helpless, weighing only about 300–700 g (10–25 oz) depending on the species.

The mother stays in a lowered metabolic state but wakes enough to clean and nurse her cubs. She produces rich milk without eating anything herself, so a mother polar bear may go about eight months without food from the time she enters the den until she returns to the sea ice to hunt. By spring, the cubs are strong enough to follow her outside.

Which bears do not hibernate?

Not every bear dens. Male polar bears and females without cubs stay active all winter, because the sea ice is where they hunt seals. Tropical bears such as the sun bear, sloth bear and spectacled bear have food all year, so they never hibernate. The giant panda eats bamboo through the winter and does not hibernate either.

Food matters more than cold. Black bears that find steady food near towns, or a mild winter with acorns still on the ground, may den later or for shorter periods. For other winter sleepers, from bats to hedgehogs, see our list of animals that hibernate.