As winter approaches,brown bears—often called grizzly bears—prepare for a long hibernation. During the fall, a brown bear eats practically around the clock, stocking up for the four to seven months when it’ll have to live off stored body fat. A grizzly may chow down on 90 pounds (40 kilograms) of food each day.
As the cold swoops in, the fattened bear waddles into a den among rocks or one it dug out among tree roots. As it falls into a deep sleep, the bear’s heart rate drops from about 40 beats a minute to as low as 8 beats a minute. All the bear’s body functions slow down.
The female brown bear enters her den pregnant with one (sometimes two or three) baby bears. If she succeeded in finding enough food to have a healthy store of fat, the embryo, or tiny developing baby bear, continues to develop and is born after a couple of months. If the mother didn’t fatten up enough, the embryo might not develop.
“Mama bear” doesn’t even wake up as her blind and hairless cub is born midwinter. The tiny bear, about the size of a chipmunk, is just strong enough to crawl into a position where it settles in to nurse. A female brown bear’s milk is very rich in fat and calories, so the cub grows quickly. By the time the adult grizzly wakes up in the spring, her baby is strong enough to follow her out of the den.
Nearly half of all brown bear cubs born are likely to die before they’re a year old. Some die of disease, and others die of starvation. Predators such as wolves, mountain lions, and adult male bears—even a cub’s own father—are threats, especially to cubs that are separated from their mothers. But mother brown bears are fiercely protective, so many cubs do survive. They live with their mothers for up to three years, and then they’re usually ready to face life on their own.